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/app-server-protocol/src/protocol/v2/account.rs

@@ -41,6 +41,11 @@ pub enum Account {
         #[serde(default)]
         uses_codex_managed_credentials: bool,
     },
+
+    /// Google Cloud Vertex AI with externally managed credentials.
+    #[serde(rename = "gcp", rename_all = "camelCase")]
+    #[ts(rename = "gcp", rename_all = "camelCase")]
+    Gcp {},
 }
 
 impl From<ProviderAccount> for Account {
@@ -53,6 +58,7 @@ impl From<ProviderAccount> for Account {
             } => Self::AmazonBedrock {
                 uses_codex_managed_credentials,
             },
+            ProviderAccount::Gcp => Self::Gcp {},
         }
     }
 }