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

@@ -1674,6 +1674,7 @@ mod tests {
     use super::*;
     use anyhow::Result;
     use codex_protocol::ThreadId;
+    use codex_protocol::account::AmazonBedrockCredentialSource;
     use codex_protocol::account::PlanType;
     use codex_protocol::models::BUILT_IN_PERMISSION_PROFILE_READ_ONLY;
     use codex_protocol::parse_command::ParsedCommand;
@@ -2777,6 +2778,41 @@ mod tests {
             serde_json::to_value(&chatgpt)?,
         );
 
+        let codex_managed_bedrock = v2::Account::AmazonBedrock {
+            credential_source: AmazonBedrockCredentialSource::CodexManaged,
+        };
+        assert_eq!(
+            json!({
+                "type": "amazonBedrock",
+                "credentialSource": "codexManaged",
+            }),
+            serde_json::to_value(&codex_managed_bedrock)?,
+        );
+
+        let aws_managed_bedrock = v2::Account::AmazonBedrock {
+            credential_source: AmazonBedrockCredentialSource::AwsManaged,
+        };
+        assert_eq!(
+            json!({
+                "type": "amazonBedrock",
+                "credentialSource": "awsManaged",
+            }),
+            serde_json::to_value(&aws_managed_bedrock)?,
+        );
+
+        Ok(())
+    }
+
+    #[test]
+    fn account_defaults_legacy_bedrock_credential_source() -> Result<()> {
+        assert_eq!(
+            v2::Account::AmazonBedrock {
+                credential_source: AmazonBedrockCredentialSource::AwsManaged,
+            },
+            serde_json::from_value(json!({
+                "type": "amazonBedrock",
+            }))?,
+        );
         Ok(())
     }