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

sdk/python/src/openai_codex/generated/v2_all.py

@@ -43,6 +43,13 @@ class AmazonBedrockAccount(BaseModel):
     ] = False
 
 
+class GcpAccount(BaseModel):
+    model_config = ConfigDict(
+        populate_by_name=True,
+    )
+    type: Annotated[Literal["gcp"], Field(title="GcpAccountType")]
+
+
 class AccountRoutingOverride(Enum):
     no_constraint = "NO_CONSTRAINT"
     us = "us"
@@ -6847,11 +6854,11 @@ class ChatgptAccount(BaseModel):
     type: Annotated[Literal["chatgpt"], Field(title="ChatgptAccountType")]
 
 
-class Account(RootModel[ApiKeyAccount | ChatgptAccount | AmazonBedrockAccount]):
+class Account(RootModel[ApiKeyAccount | ChatgptAccount | AmazonBedrockAccount | GcpAccount]):
     model_config = ConfigDict(
         populate_by_name=True,
     )
-    root: ApiKeyAccount | ChatgptAccount | AmazonBedrockAccount
+    root: ApiKeyAccount | ChatgptAccount | AmazonBedrockAccount | GcpAccount
 
 
 class AccountLoginCompletedNotification(BaseModel):