feat: expose AWS account state from account/read (#19048)

## Why

AWS/Bedrock mode currently reports `account: null` with
`requiresOpenaiAuth: false` from `account/read`. That suppresses the
OpenAI-auth requirement, but it does not let app clients distinguish AWS
auth from any other non-OpenAI custom provider. For the prototype AWS
provider UX, clients need a simple provider-derived signal so they can
suppress ChatGPT/API-key login and token-refresh paths without
hardcoding Bedrock checks.

## What changed

- Adds an `aws` variant to the v2 `Account` protocol union.
- Adds `ProviderAccountKind` to `codex-model-provider` so the runtime
provider owns the app-visible account classification.
- Makes Amazon Bedrock return `ProviderAccountKind::Aws` from the
model-provider layer.
- Updates app-server `account/read` to map `ProviderAccountKind` to the
existing `GetAccountResponse` wire shape.
- Preserves the existing `account: null, requiresOpenaiAuth: false`
behavior for other non-OpenAI providers.
- Regenerates the app-server protocol schema fixtures.
- Adds coverage for provider account classification and for the Amazon
Bedrock `account/read` response.

## Testing

- `cargo test -p codex-model-provider`
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-app-server get_account_with_aws_provider`

## Notes

I attempted `just bazel-lock-update` and `just bazel-lock-check`, but
both are blocked in my local environment because `bazel` is not
installed.

codex-rs/model-provider/src/lib.rs

@@ -7,6 +7,10 @@ pub use auth::auth_provider_from_auth;
 pub use auth::unauthenticated_auth_provider;
 pub use bearer_auth_provider::BearerAuthProvider;
 pub use bearer_auth_provider::BearerAuthProvider as CoreAuthProvider;
+pub use codex_protocol::account::ProviderAccount;
 pub use provider::ModelProvider;
+pub use provider::ProviderAccountError;
+pub use provider::ProviderAccountResult;
+pub use provider::ProviderAccountState;
 pub use provider::SharedModelProvider;
 pub use provider::create_model_provider;