[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/schema/json/codex_app_server_protocol.schemas.json

@@ -5847,6 +5847,14 @@
           },
           {
             "properties": {
+              "credentialSource": {
+                "allOf": [
+                  {
+                    "$ref": "#/definitions/v2/AmazonBedrockCredentialSource"
+                  }
+                ],
+                "default": "awsManaged"
+              },
               "type": {
                 "enum": [
                   "amazonBedrock"
@@ -6166,6 +6174,13 @@
       "AgentPath": {
         "type": "string"
       },
+      "AmazonBedrockCredentialSource": {
+        "enum": [
+          "codexManaged",
+          "awsManaged"
+        ],
+        "type": "string"
+      },
       "AnalyticsConfig": {
         "additionalProperties": true,
         "properties": {