feat(event_handler): allow custom serializer on BedrockAgentResolver

[kimnamu]

Issue number: closes #8270

Summary

Changes

BedrockAgentResolver is the only OpenAPI resolver that doesn't expose serializer, so non-ASCII Bedrock Agent responses can't avoid \uXXXX escaping. This exposes the parent ApiGatewayResolver's existing serializer argument on BedrockAgentResolver and passes it through — matching the sibling BedrockAgentFunctionResolver, which already accepts serializer (and the approach the maintainer endorsed in #7776 / #6394).

User experience

Item	Before	After
BedrockAgentResolver(serializer=...)	❌ TypeError: unexpected keyword 'serializer'	✅ accepted
Korean/emoji response body	❌ 잔액... escaped	✅ 잔액은 1,000원입니다 💰 (with ensure_ascii=False)
Default behavior (no serializer)	escapes non-ASCII	✅ unchanged
Public API / other args / response shape	—	✅ unchanged (additive optional kwarg)

Out of scope (follow-up): defaulting the builder to ensure_ascii=False (a behavior change) — intentionally not done here.

Checklist

• Tests: 2 new GIVEN/WHEN/THEN tests in _pydantic/test_bedrock_agent.py (default escapes; custom partial(json.dumps, ensure_ascii=False) preserves non-ASCII). Reverting the source change (keeping tests) → TypeError: ... unexpected keyword argument 'serializer'; restored → 17 passed. ruff/mypy clean.

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[boring-cyborg]

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.58%. Comparing base (c0804d4) to head (27c13bc).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8271   +/-   ##
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  Coverage    96.58%   96.58%           
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  Files          286      286           
  Lines        14294    14294           
  Branches      1192     1192           
========================================
  Hits         13806    13806           
  Misses         357      357           
  Partials       131      131           

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[leandrodamascena]

Thanks @kimnamu! Approved!

[boring-cyborg]

Awesome work, congrats on your first merged pull request and thank you for helping improve everyone's experience!