feat: add custom headers support for API requests

[GregBrimble]

Add support for custom headers in API requests via:

• custom_headers field in ModelProviderInfo config
• CODEX_CUSTOM_HEADERS environment variable with format "key: value\nkey2: value2"

Headers from both sources are merged, with env var taking precedence. Both Chat Completions and Responses APIs now include custom headers.

Resolves #1152

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[GregBrimble]

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[bolinfest]

To date, we generally favor configuration via config.toml and the -c command-line flag rather than environment variables. Is there a compelling reason why environment variables should be supported for this option?

[GregBrimble]

Nope! No strong reason at all — I was just taking inspiration from how Claude Code works. But I'm more than happy to go config-only if that's what y'all prefer. I'll update this PR.

[bolinfest]

Actually, it turns out that we have some logic of our own that needs this behavior:

codex/codex-cli/src/utils/agent/agent-loop.ts

Line 328 in bd5a9e8

...(OPENAI_PROJECT ? { "OpenAI-Project": OPENAI_PROJECT } : {}),

I'm going to go ahead and add two separate fields to ModelProviderInfo:

• http_headers
• env_key_http_headers

The former will be used for hardcoded values; the latter will be used for values that should come from environment variables.

[bolinfest]

@GregBrimble see #1473

[bolinfest]

This functionality is now available in the 0.3.0 release.

[bolinfest]

@GregBrimble thanks for the push+inspiration to make this happen!