Add a built-in Google Cloud Vertex AI provider (#56265)

## What changed

- Support `model_provider = "gcp"` with a Vertex AI endpoint derived from `GOOGLE_CLOUD_PROJECT` and `GOOGLE_CLOUD_LOCATION`, defaulting to the `global` location.
- Obtain credentials through `gcloud auth application-default print-access-token` by default, and allow `base_url`, `auth`, and `http_headers` overrides under `model_providers.gcp`.
- Use a static model catalog tailored to Vertex AI, preserve function tools, namespaces, and client-side tool search, and support remote compaction V2. Disable unsupported reasoning summaries, Responses Lite, custom tools, web search, and image generation.
- Expose a `gcp` account type through the app-server protocol and generated clients without resolving credentials during account reads or requiring OpenAI authentication.

## Testing

Add coverage for endpoint resolution, configuration overrides, model catalog filtering, authenticated requests, unsupported request fields, missing credentials, account reporting across provider changes, and automatic remote compaction.

GitOrigin-RevId: 97a3158269e6dbc2605e2cf576998e09817ea12a

codex-rs/exec/src/daybreak.rs

@@ -73,7 +73,7 @@ pub(crate) async fn program_for_turn(
                 Some(Account::ApiKey { .. }) => {
                     features.enabled(Feature::ApiKeyCyberAccessPrograms)
                 }
-                Some(Account::AmazonBedrock { .. }) | None => false,
+                Some(Account::AmazonBedrock { .. } | Account::Gcp {}) | None => false,
             };
             return Ok(eligible.then_some(standard.into()));
         }