fix: Use locale-invariant toLowerCase for header normalization

[anishesg]

Changes being requested

This PR fixes a critical bug where the SDK crashes on systems running with Turkish locale (tr-TR) on Windows. The root cause is in src/internal/headers.ts line 77, where name.toLowerCase() performs locale-sensitive string conversion. On Turkish locales, uppercase 'I' converts to dotless 'ı' instead of 'i', producing invalid HTTP header names like 'openaı-organization' that violate HTTP token validation in Node.js's underlying network layer (undici).

The fix replaces name.toLowerCase() with name.toLocaleLowerCase('en-US') to ensure locale-invariant header normalization that always produces ASCII-compliant header names regardless of system locale. This is a minimal one-line change that aligns with HTTP specification requirements.

I've also added test cases that verify header names containing 'I' (like 'OpenAI-Organization') are normalized correctly and that case-insensitive header merging works as expected.

Additional context & links

This issue affects Turkish Windows users and potentially users of other locales with non-standard case mappings. The Turkish 'I' problem is a well-known locale-sensitivity issue in JavaScript string operations.

Fixes #1928

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 4902cbd4e7

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Expect normalized header names in the new test

This expected value does not match what buildHeaders returns for object inputs: it ultimately stores the entry in a WHATWG Headers, and Headers.append('OpenAI-Organization', ...) normalizes the name to openai-organization on iteration. As written, this new case fails on the standard Node Headers implementation even though the product code lowercases the header name correctly.

Useful? React with 👍 / 👎.

[anishesg]

ah good point, the test should expect the lowercased version since that's what Headers actually returns when you iterate

[anishesg]

ah good point, the test should expect the lowercased version since that's what Headers actually returns when you iterate