fix(sentry-okhttp): Skip synthetic 504 for FORCE_CACHE cache misses (#5299)

* fix(sentry-okhttp): Skip synthetic 504 for FORCE_CACHE cache misses

OkHttp's CacheInterceptor synthesizes a 504 "Unsatisfiable Request"
response when a request with `only-if-cached` (e.g. CacheControl.FORCE_CACHE)
misses the cache. This is not a real server error, so don't capture it
as a failed request event.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: Update changelog PR link to #5299

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>