[codex] expose Bedrock credential source in account/read (#27751)

## Why

`account/read` currently reports only `type: "amazonBedrock"`, so
clients cannot distinguish a Codex-managed Bedrock API key from
credentials supplied by AWS. The app UI needs that distinction to render
the appropriate account state without duplicating provider-auth logic.

Credential-source selection belongs to the Bedrock model provider
because it already owns the precedence between managed Bedrock auth and
the external AWS credential path. This builds on #27443 and #27689.

## What changed

- Added `AmazonBedrockCredentialSource` with `codexManaged` and
`awsManaged` values.
- Included the selected credential source in
`ProviderAccount::AmazonBedrock` and the app-server `Account` response.
- Made `AmazonBedrockModelProvider::account_state()` classify the source
from its managed-auth state.
- Regenerated the app-server JSON and TypeScript schemas.
- Updated app-server account documentation and downstream TUI matches.

`codexManaged` means the provider found a managed Bedrock API key.
`awsManaged` identifies the provider's external AWS credential path; it
does not assert that the AWS credential chain has been validated.

## Testing

- Added model-provider coverage for Codex-managed precedence and
AWS-managed fallback.
- Added app-server protocol serialization coverage for both wire values.
- Added app-server integration coverage for both `account/read`
responses.
- `just test -p codex-protocol -p codex-model-provider -p
codex-app-server-protocol` (497 tests passed).

After rebasing onto #27711, the `codex-app-server` test target compiled
past the image-generation `PathUri` migration. Local linking was then
interrupted by disk exhaustion (`No space left on device`).

codex-rs/protocol/src/account.rs

@@ -34,8 +34,21 @@ pub enum PlanType {
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum ProviderAccount {
     ApiKey,
-    Chatgpt { email: String, plan_type: PlanType },
-    AmazonBedrock,
+    Chatgpt {
+        email: String,
+        plan_type: PlanType,
+    },
+    AmazonBedrock {
+        credential_source: AmazonBedrockCredentialSource,
+    },
+}
+
+#[derive(Serialize, Deserialize, Copy, Clone, Debug, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(rename_all = "camelCase")]
+pub enum AmazonBedrockCredentialSource {
+    CodexManaged,
+    AwsManaged,
 }
 
 impl PlanType {