fix(schema): retry transient network errors when downloading schemas

[kddejong]

Description

Schema specs are downloaded from a GitHub release asset (resource-provider-enhanced-schemas → schemas-cfn-lint.zip) served through a CDN that intermittently resets connections under load (Remote end closed connection without response). The download had no retry, so a single reset failed the whole cfn-lint --update-specs run. In CI the unitlint matrix fires ~18 concurrent downloads with fail-fast: true, so one reset cancels every job in the run.

This wraps the network calls in the URL helpers with a small shared retry/backoff helper:

• 3 attempts, 0.5s base delay with exponential backoff
• Retries connection-level errors (URLError, ConnectionResetError / RemoteDisconnected, timeouts — all OSError) and HTTP 429/5xx
• Deterministic HTTP 4xx responses are not retried (fail fast)
• Applied to get_url_retrieve (schema zip), get_url_content (version.json), and url_has_newer_version (HEAD check)

This reduces transient CI flakiness; it does not make the download bulletproof against a sustained outage.

Additional change: FindInMap guardrail test

Follow-up to #4628 (which made Fn::FindInMap maxItems unconditionally 4 so DefaultValue works without AWS::LanguageExtensions). Adds a regression test asserting that a 4th element which is not a {DefaultValue: ...} object is still rejected via prefixItems[3] (is not of type 'object') even without the transform — locking in that relaxing the arity did not weaken the 4th-element shape check.

Validation

• pytest test/unit/module/helpers -q and pytest test/unit/rules/functions/test_find_in_map.py -q
• ruff check / ruff format --check on the changed files
• mypy src/cfnlint/helpers.py
• cfn-lint --update-specs --force succeeds locally

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.58%. Comparing base (732ea3c) to head (56e981b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4633   +/-   ##
=======================================
  Coverage   94.57%   94.58%           
=======================================
  Files         432      432           
  Lines       15395    15417   +22     
  Branches     2984     2986    +2     
=======================================
+ Hits        14560    14582   +22     
  Misses        456      456           
  Partials      379      379           

Flag	Coverage Δ
unittests	94.58% <100.00%> (+<0.01%)	⬆️

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