feat: add a built-in Amazon Bedrock model provider (#18744)

## Why

Codex needs a first-class `amazon-bedrock` model provider so users can
select Bedrock without copying a full provider definition into
`config.toml`. The provider has Codex-owned defaults for the pieces that
should stay consistent across users: the display `name`, Bedrock
`base_url`, and `wire_api`.

At the same time, users still need a way to choose the AWS credential
profile used by their local environment. This change makes
`amazon-bedrock` a partially modifiable built-in provider: code owns the
provider identity and endpoint defaults, while user config can set
`model_providers.amazon-bedrock.aws.profile`.

For example:

```toml
model_provider = "amazon-bedrock"

[model_providers.amazon-bedrock.aws]
profile = "codex-bedrock"
```

## What Changed

- Added `amazon-bedrock` to the built-in model provider map with:
  - `name = "Amazon Bedrock"`
  - `base_url = "https://bedrock-mantle.us-east-1.api.aws/v1"`
  - `wire_api = "responses"`
- Added AWS provider auth config with a profile-only shape:
`model_providers.<id>.aws.profile`.
- Kept AWS auth config restricted to `amazon-bedrock`; custom providers
that set `aws` are rejected.
- Allowed `model_providers.amazon-bedrock` through reserved-provider
validation so it can act as a partial override.
- During config loading, only `aws.profile` is copied from the
user-provided `amazon-bedrock` entry onto the built-in provider. Other
Bedrock provider fields remain hard-coded by the built-in definition.
- Updated the generated config schema for the new provider AWS profile
config.

codex-rs/core/tests/suite/client.rs

@@ -841,6 +841,7 @@ async fn send_provider_auth_request(server: &MockServer, auth: ModelProviderAuth
         env_key_instructions: None,
         experimental_bearer_token: None,
         auth: Some(auth),
+        aws: None,
         wire_api: WireApi::Responses,
         query_params: None,
         http_headers: None,
@@ -2132,6 +2133,7 @@ async fn azure_responses_request_includes_store_and_reasoning_ids() {
         env_key_instructions: None,
         experimental_bearer_token: None,
         auth: None,
+        aws: None,
         wire_api: WireApi::Responses,
         query_params: None,
         http_headers: None,
@@ -2750,6 +2752,7 @@ async fn azure_overrides_assign_properties_used_for_responses_url() {
         env_key: Some(EXISTING_ENV_VAR_WITH_NON_EMPTY_VALUE.to_string()),
         experimental_bearer_token: None,
         auth: None,
+        aws: None,
         query_params: Some(std::collections::HashMap::from([(
             "api-version".to_string(),
             "2025-04-01-preview".to_string(),
@@ -2841,6 +2844,7 @@ async fn env_var_overrides_loaded_auth() {
         env_key_instructions: None,
         experimental_bearer_token: None,
         auth: None,
+        aws: None,
         wire_api: WireApi::Responses,
         http_headers: Some(std::collections::HashMap::from([(
             "Custom-Header".to_string(),