feat(ai): openai-codex-responses support use custom apiKey and baseUrl

[CyJaySong]

What

Add support for API-key based custom providers on the OpenAI Codex Responses path.

This change lets custom openai-codex-responses providers use bearer API keys while preserving the first-party Codex request shape, including Codex-style response metadata, cache-related headers, and the existing OMP originator / User-Agent behavior.

Why

Some Codex-compatible gateways route requests correctly only when the request follows the Codex Responses API contract closely. API-key based custom providers previously could not fully use that path, which made these gateways fall back to unintended model routing.

Testing

• Ran bun --cwd=packages/ai run check
• Ran git diff --check
• Added/updated provider streaming coverage for API-key Codex Responses requests

---

• bun check passes
• Tested locally
• CHANGELOG updated (if user-facing)

[chatgpt-codex-connector]

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[roboomp]

P3: the feature is scoped to Codex Responses custom gateways, but the diff is currently broken/off-contract.

Headline blockers: packages/ai changed tests fail (bun --cwd=packages/ai test test/openai-codex-stream.test.ts: 53 pass / 4 fail); the new metadata helper does not implement the project/projectless workspace contract; and the opaque-key beta-header contract is now inconsistent across old and new coverage. Also needs changelog entries for the user-facing packages/ai and packages/coding-agent changes.

Thanks for pushing the gateway path forward; once the failing contracts are reconciled, this should be much easier to judge on the intended behavior.

[CyJaySong]

@can1357