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.

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

codex-rs/Cargo.toml

@@ -92,6 +92,7 @@ members = [
     "thread-store",
     "codex-experimental-api-macros",
     "plugin",
+    "model-provider",
 ]
 resolver = "2"
 
@@ -154,6 +155,7 @@ codex-network-proxy = { path = "network-proxy" }
 codex-ollama = { path = "ollama" }
 codex-otel = { path = "otel" }
 codex-plugin = { path = "plugin" }
+codex-model-provider = { path = "model-provider" }
 codex-process-hardening = { path = "process-hardening" }
 codex-protocol = { path = "protocol" }
 codex-realtime-webrtc = { path = "realtime-webrtc" }