fix: handle ByteString error in embedder validation flow

[roomote]

Related GitHub Issue

Closes: #10973

Description

This PR attempts to address the ByteString conversion error that occurs when non-ASCII characters (e.g., Cyrillic) are present in the API key or base URL configuration for the codebase indexer.

Root Cause: HTTP headers only support Latin-1 characters (values 0-255). When the OpenAI SDK or fetch API tries to set headers containing characters with values > 255, it throws the ByteString error.

Implementation:

• Added ByteString error detection in handleValidationError function in src/services/code-index/shared/validation-helpers.ts
• Returns a user-friendly localized error message using the existing common:errors.api.invalidKeyInvalidChars translation key
• Added comprehensive test coverage for the new error handling and related validation scenarios

This approach is consistent with the existing ByteString error handling in src/api/providers/utils/error-handler.ts.

Feedback and guidance are welcome.

Test Procedure

1. Run the validation-helpers tests:

   cd src && npx vitest run services/code-index/shared/__tests__/validation-helpers.spec.ts

2. All 26 tests pass, including new tests for:
   • ByteString conversion error handling
   • Connection errors (ECONNREFUSED, ENOTFOUND, ETIMEDOUT)
   • HTTP status code handling (401, 404, 429)
   • Custom handler support

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New and/or updated tests have been added to cover my changes.
• Documentation Impact: No documentation updates are required.
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

• No documentation updates are required.

Additional Notes

The translation key common:errors.api.invalidKeyInvalidChars already exists in all supported locales, so no new translations were needed.

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Review complete. No issues found.

The implementation correctly handles ByteString conversion errors in the embedder validation flow, consistent with existing error handling patterns. Tests pass and translation keys are in place across all locales.

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