fix: include error response body and status code in FalException

[otopba]

Summary

responseToException() in HttpClient parsed the JSON error body but silently discarded it, always creating FalException with only "Request failed with code: NNN". This lost valuable error details returned by the fal.ai API.

Before:

FalException: Request failed with code: 422

After:

FalException: Request failed with code: 422: The prompt may contain intellectual property references that cannot be processed.

Changes

• FalException — added statusCode (int) and body (String) fields with backward-compatible constructors
• HttpClient.responseToException() — now reads the response body and extracts human-readable messages from common error formats:
  • {"detail": [{"msg": "...", ...}]} — validation errors (messages joined with ; )
  • {"detail": "..."} — string detail
  • {"message": "..."} — generic error message
  • {"error": "..."} — error string
• HttpClient.extractDetailMessage() — safe JSON parsing with fallback (never throws)
• 23 unit tests covering all parsing paths, edge cases (invalid JSON, null fields, non-string primitives, empty arrays), backward compatibility, and requestId preservation

Backward Compatibility

• All existing FalException constructors preserved unchanged
• getMessage() still starts with "Request failed with code: NNN" — detail is appended after ": "
• New fields default to -1 / null when using legacy constructors
• No changes to client-async or client-kotlin modules (they use HttpClient internally)

Test plan

• All 23 unit tests pass
• All three modules (client, client-async, client-kotlin) compile successfully
• Verified with JDK 17 (Corretto)

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