feat: add managed Bedrock login API (#31327)

## Why

App-server clients can report whether Amazon Bedrock is using
AWS-managed credentials or a Codex-managed API key, but they do not have
a matching API for creating the managed login. This PR defines that
experimental wire contract independently from its implementation.

Managed Bedrock API keys are already a primary `CodexAuth` mode. The API
therefore describes a normal Codex login that replaces the current
stored auth rather than introducing provider-scoped credential storage.

## What changed

- Add the experimental `amazonBedrock` variant to `account/login/start`.
- Accept an API key and AWS region and return a matching discriminated
response.
- Gate the request behind the app-server `experimentalApi` capability.
- Regenerate the JSON and TypeScript protocol schemas.
- Document the login contract, notifications, primary-auth replacement
semantics, restart boundary, and non-transactional durable writes.

## Impact

This PR defines the API shape but does not implement login behavior. The
next PR adds validation, persistence through the existing Codex auth
lifecycle, provider selection, and notifications.

## Validation

- `just test -p codex-app-server-protocol`

## Stack

1. **#31327 Managed Bedrock experimental API** — base: `main`
2. #31326 Managed Bedrock login — base: `codex/managed-bedrock-api`
3. #31325 Managed Bedrock logout — base:
`codex/managed-bedrock-login-v2`

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -2722,6 +2722,34 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn serialize_account_login_amazon_bedrock() -> Result<()> {
+        let request = ClientRequest::LoginAccount {
+            request_id: RequestId::Integer(2),
+            params: v2::LoginAccountParams::AmazonBedrock {
+                api_key: "secret".to_string(),
+                region: "us-west-2".to_string(),
+            },
+        };
+        assert_eq!(
+            json!({
+                "method": "account/login/start",
+                "id": 2,
+                "params": {
+                    "type": "amazonBedrock",
+                    "apiKey": "secret",
+                    "region": "us-west-2"
+                }
+            }),
+            serde_json::to_value(&request)?,
+        );
+        assert_eq!(
+            json!({"type": "amazonBedrock"}),
+            serde_json::to_value(v2::LoginAccountResponse::AmazonBedrock {})?,
+        );
+        Ok(())
+    }
+
     #[test]
     fn serialize_account_login_chatgpt() -> Result<()> {
         let request = ClientRequest::LoginAccount {