feat: use thread-local curl handle for multi-concurrency support

[maxday]

Description of changes:

• Replace the per-instance CURL* member with a thread_local static handle, enabling safe concurrent invocations across multiple threads in multi-concurrency mode. Each thread gets its own curl handle, initialized once and reused across runtime instances on that thread.
• Remove curl handle lifecycle management from the runtime class — the handle is now created at thread start and cleaned up automatically on thread exit.
• Fix clang-tidy warnings surfaced by the newer CI toolchain: add const qualifiers, use = default destructor, make set_curl_post_result_options static, use std::uint8_t base for verbosity enum, move internal helper to anonymous namespace, and add missing direct
  includes.
• Add unit tests covering thread-local behavior (concurrent runtimes on different threads, multiple runtimes on same thread) and existing public API surface (invocation_response, http::response, outcome, invocation_request).
• Remove no-op placeholder test. No longer needed now that real unit tests are in place.

Test result - GitHub action is ✅
Test result - locally:

Test project /local/home/maxday/cpp-analysis/aws-lambda-cpp/build
      Start  1: ThreadLocalCurl.runtime_construction_succeeds
 1/33 Test  #1: ThreadLocalCurl.runtime_construction_succeeds ..................................   Passed    0.01 sec
      Start  2: ThreadLocalCurl.multiple_runtimes_on_same_thread_do_not_crash
 2/33 Test  #2: ThreadLocalCurl.multiple_runtimes_on_same_thread_do_not_crash ..................   Passed    0.01 sec
      Start  3: ThreadLocalCurl.concurrent_runtimes_on_different_threads
 3/33 Test  #3: ThreadLocalCurl.concurrent_runtimes_on_different_threads .......................   Passed    0.01 sec
      Start  4: ThreadLocalCurl.sequential_requests_on_same_runtime
 4/33 Test  #4: ThreadLocalCurl.sequential_requests_on_same_runtime ............................   Passed    0.01 sec
      Start  5: InvocationResponseTest.success_response_has_correct_payload_and_content_type
 5/33 Test  #5: InvocationResponseTest.success_response_has_correct_payload_and_content_type ...   Passed    0.00 sec
      Start  6: InvocationResponseTest.success_response_with_json
 6/33 Test  #6: InvocationResponseTest.success_response_with_json ..............................   Passed    0.00 sec
      Start  7: InvocationResponseTest.success_response_with_empty_payload
 7/33 Test  #7: InvocationResponseTest.success_response_with_empty_payload .....................   Passed    0.00 sec
      Start  8: InvocationResponseTest.failure_response_is_not_success
 8/33 Test  #8: InvocationResponseTest.failure_response_is_not_success .........................   Passed    0.00 sec
      Start  9: InvocationResponseTest.failure_response_contains_error_message
 9/33 Test  #9: InvocationResponseTest.failure_response_contains_error_message .................   Passed    0.00 sec
      Start 10: InvocationResponseTest.failure_response_json_escapes_quotes
10/33 Test #10: InvocationResponseTest.failure_response_json_escapes_quotes ....................   Passed    0.00 sec
      Start 11: InvocationResponseTest.failure_response_json_escapes_backslash
11/33 Test #11: InvocationResponseTest.failure_response_json_escapes_backslash .................   Passed    0.00 sec
      Start 12: InvocationResponseTest.failure_response_json_escapes_newlines
12/33 Test #12: InvocationResponseTest.failure_response_json_escapes_newlines ..................   Passed    0.00 sec
      Start 13: InvocationResponseTest.failure_response_json_escapes_tabs
13/33 Test #13: InvocationResponseTest.failure_response_json_escapes_tabs ......................   Passed    0.00 sec
      Start 14: InvocationResponseTest.failure_response_json_escapes_control_characters
14/33 Test #14: InvocationResponseTest.failure_response_json_escapes_control_characters ........   Passed    0.00 sec
      Start 15: InvocationResponseTest.success_response_with_binary_content_type
15/33 Test #15: InvocationResponseTest.success_response_with_binary_content_type ...............   Passed    0.00 sec
      Start 16: InvocationResponseTest.constructor_based_failure
16/33 Test #16: InvocationResponseTest.constructor_based_failure ...............................   Passed    0.00 sec
      Start 17: HttpResponseTest.add_and_retrieve_header
17/33 Test #17: HttpResponseTest.add_and_retrieve_header .......................................   Passed    0.00 sec
      Start 18: HttpResponseTest.headers_are_lowercased
18/33 Test #18: HttpResponseTest.headers_are_lowercased ........................................   Passed    0.00 sec
      Start 19: HttpResponseTest.has_header_returns_false_for_missing
19/33 Test #19: HttpResponseTest.has_header_returns_false_for_missing ..........................   Passed    0.00 sec
      Start 20: HttpResponseTest.append_body_accumulates
20/33 Test #20: HttpResponseTest.append_body_accumulates .......................................   Passed    0.00 sec
      Start 21: HttpResponseTest.append_body_empty
21/33 Test #21: HttpResponseTest.append_body_empty .............................................   Passed    0.00 sec
      Start 22: HttpResponseTest.set_response_code
22/33 Test #22: HttpResponseTest.set_response_code .............................................   Passed    0.00 sec
      Start 23: HttpResponseTest.multiple_headers
23/33 Test #23: HttpResponseTest.multiple_headers ..............................................   Passed    0.00 sec
      Start 24: OutcomeTest.success_outcome
24/33 Test #24: OutcomeTest.success_outcome ....................................................   Passed    0.00 sec
      Start 25: OutcomeTest.failure_outcome
25/33 Test #25: OutcomeTest.failure_outcome ....................................................   Passed    0.00 sec
      Start 26: OutcomeTest.move_success
26/33 Test #26: OutcomeTest.move_success .......................................................   Passed    0.00 sec
      Start 27: OutcomeTest.move_failure
27/33 Test #27: OutcomeTest.move_failure .......................................................   Passed    0.00 sec
      Start 28: OutcomeTest.with_response_code
28/33 Test #28: OutcomeTest.with_response_code .................................................   Passed    0.00 sec
      Start 29: InvocationRequestTest.get_time_remaining_future_deadline
29/33 Test #29: InvocationRequestTest.get_time_remaining_future_deadline .......................   Passed    0.00 sec
      Start 30: InvocationRequestTest.get_time_remaining_past_deadline
30/33 Test #30: InvocationRequestTest.get_time_remaining_past_deadline .........................   Passed    0.00 sec
      Start 31: InvocationRequestTest.default_fields_are_empty
31/33 Test #31: InvocationRequestTest.default_fields_are_empty .................................   Passed    0.00 sec
      Start 32: VersionTest.version_string_not_empty
32/33 Test #32: VersionTest.version_string_not_empty ...........................................   Passed    0.00 sec
      Start 33: VersionTest.version_format
33/33 Test #33: VersionTest.version_format .....................................................   Passed    0.00 sec

100% tests passed, 0 tests failed out of 33

Label Time Summary:
unit    =   0.15 sec*proc (33 tests)

Total Test time (real) =   0.17 sec

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.