fix(api): return proper search error status codes

[SaitejaKommi]

Summary

This PR refines Search API error handling so known client-side query errors return 400, Elasticsearch availability errors return 503, and unknown errors continue to return 500.

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Related issue

• Fixes: Search API should return 400/503 for known error classes instead of always 500 #375

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What changed

• Updated query exception handling in search.py
• Added error-title-based HTTP status mapping in api.py
• Added unit tests in test_api.py covering:
  • 400 Bad Request
  • 503 Service Unavailable
  • 500 Internal Server Error

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Behavior

Before

• Most non-success search errors returned 500

After

• Invalid query / validation / parsing errors → 400
• Elasticsearch connectivity / upstream failures → 503
• Unknown or unclassified errors → 500

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Why this is needed

• Improves API correctness and client expectations
• Enables better retry/error handling for consumers
• Distinguishes client errors from infrastructure failures
• Preserves existing response schema

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Testing Done

Backend unit tests

• 45 passed

Backend integration tests

• 73 passed
• 3 xfailed

Frontend tests

• All test files passed in dev/prod modes

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