feat: add opt-in provider runtime abstraction (#17713)

## Summary

- Add `codex-model-provider` as the runtime home for model-provider
behavior that does not belong in `codex-core`, `codex-login`, or
`codex-api`.
- The new crate wraps configured `ModelProviderInfo` in a
`ModelProvider` trait object that can resolve the API provider config,
provider-scoped auth manager, and request auth provider for each call.
- This centralizes provider auth behavior in one place today, and gives
us an extension point for future provider-specific auth, model listing,
request setup, and related runtime behavior.

## Tests
Ran tests manually to make sure that provider auth under different
configs still work as expected.

---------

Co-authored-by: pakrym-oai <pakrym@openai.com>

codex-rs/core/tests/suite/client.rs

@@ -234,7 +234,7 @@ move /y tokens.next tokens.txt >nul
         ModelProviderAuthInfo {
             command: self.command.clone(),
             args: self.args.clone(),
-            // Match the provider-auth default to avoid brittle shell-startup timing in CI.
+            // Match the model-provider default to avoid brittle shell-startup timing in CI.
             timeout_ms: non_zero_u64(/*value*/ 5_000),
             refresh_interval_ms: 60_000,
             cwd: match codex_utils_absolute_path::AbsolutePathBuf::try_from(self.tempdir.path()) {