test: address coderabbit feedback on PR #1259 review round 2

Four findings from coderabbit, all in execution/engine/ tests, all
applying the package conventions documented in CLAUDE.md.

federation_caching_root_split_test.go (the only finding we introduced):
- Removed a pointless initial ClearLog and added explicit GetLog()
  assertions for both cold-path (6 cache operations across accounts /
  products / reviews) and warm-path (all hits, no further set) per the
  cache log rule.

federation_caching_entity_field_args_test.go (pre-existing patterns
flagged retroactively by the new self-contained-subtest rule):
- Removed shared entityFieldArgsSetup struct, newEntityFieldArgsSetup
  constructor, and parent-level peekCache helper. Each subtest now
  inlines its own cache, tracker, gateway setup, ctx, and gqlClient.
- Switched all gqlClient.QueryString(...) calls to QueryStringWithHeaders.

federation_caching_source_test.go (pre-existing pattern):
- Replaced the two cachingTestQueryPath("subscriptions/...") calls with
  inline subscription documents. graphql_client_test.go's subscription
  helper accepts inline operation strings while preserving the file-path
  compatibility used by other suites.

Tests: full resolve + execution suites pass; resolve passes under -race;
gofmt + gci clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jensneuse