feat: add Bedrock API key as a managed auth mode (#27443)

## Why

Codex needs to manage Amazon Bedrock API key credentials through the
existing auth lifecycle instead of introducing a separate auth manager
or provider-specific credential file. Treating Bedrock API key login as
a primary auth mode gives it the same persistence, keyring, reload, and
logout behavior as the existing OpenAI API key and ChatGPT modes.

The credential is valid only for the `amazon-bedrock` model provider.
OpenAI-compatible providers must reject this auth mode rather than
treating the Bedrock key as an OpenAI bearer token.

## What changed

- Added `bedrockApiKey` as an app-server `AuthMode` and
`CodexAuth::BedrockApiKey` as a primary `AuthManager` mode.
- Added `BedrockApiKeyAuth`, containing the API key and AWS region, to
the existing `AuthDotJson` payload stored in `$CODEX_HOME/auth.json` or
the configured keyring backend.
- Added `login_with_bedrock_api_key(...)`, parallel to
`login_with_api_key(...)`, which replaces the current stored login with
Bedrock credentials.
- Reused generic auth reload and logout behavior instead of adding a
Bedrock-specific auth manager or logout path.
- Updated login restrictions, status reporting, diagnostics, telemetry
classification, generated app-server schemas, and auth fixtures for the
new mode.
- Added explicit errors when Bedrock API key auth is selected with an
OpenAI-compatible model provider.

This PR establishes managed storage and auth-mode behavior. Routing the
managed key and region into Amazon Bedrock requests will be in follow-up
PRs.

codex-rs/login/src/auth/storage.rs

@@ -18,6 +18,7 @@ use std::sync::Arc;
 use std::sync::Mutex;
 use tracing::warn;
 
+use super::BedrockApiKeyAuth;
 use crate::token_data::TokenData;
 use codex_agent_identity::AgentIdentityJwtClaims;
 use codex_agent_identity::decode_agent_identity_jwt;
@@ -48,6 +49,9 @@ pub struct AuthDotJson {
 
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub personal_access_token: Option<String>,
+
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub bedrock_api_key: Option<BedrockApiKeyAuth>,
 }
 
 #[derive(Deserialize, Serialize, Clone, Debug, PartialEq, Eq)]