PR Summary

Low Risk
Small refactor in resolveModelWithRegistry plus a tightened regression test; behavior should be unchanged aside from better coverage of provider override propagation.

Overview
Simplifies resolveModelWithRegistry so inline-configured models only short-circuit resolution when they explicitly specify an api, ensuring forward-compat fallbacks still apply when api is omitted.

Extends the openai-codex regression test to assert provider-level baseUrl and headers are preserved when resolving the gpt-5.x forward-compat fallback.

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Landed via rebase onto main.

• Gate: pnpm check && pnpm build && pnpm test
• Land commit: b51de84
• Merge commit: d91d24e

Follow-up cleanup for #39753.

Greptile Summary

This PR is a two-part follow-up cleanup to #39753. In model.ts, the nested if (inlineMatch) { if (inlineMatch.api) { ... } } guard is collapsed into the equivalent if (inlineMatch?.api) optional-chaining form — no behavioral change at all. In model.test.ts, the existing regression test for issue #39682 is extended to also assert that a configured baseUrl and provider-level headers correctly survive the codex forward-compat path (i.e. they are propagated through applyConfiguredProviderOverrides even when the inline model entry lacks an explicit api).

Key changes:

• model.ts line 163: if (inlineMatch?.api) replaces the two-level nested guard — semantically identical, fewer lines.
• model.test.ts: adds headers: { "X-Custom-Auth": "token-123" } to the test fixture and asserts both baseUrl and headers appear in the resolved model, giving concrete coverage for the configuration-survives-forward-compat scenario.

Confidence Score: 5/5

• Safe to merge — this is a pure refactor with no behavioral change and adds regression coverage.
• This PR contains only safe changes: a semantically identical refactor of a guard condition and extension of an existing test to verify configuration propagation. The optional-chaining simplification is correct, and the test extension directly exercises the existing applyConfiguredProviderOverrides logic, validating that headers and baseUrl correctly flow through the forward-compat path. No logic changes, no edge cases missed.
• No files require special attention.

_{Last reviewed commit: b51de84}