Add workload identity federation support to base-action (#1378)

* Add workload identity federation support to base-action

Move the workload identity module into base-action so the standalone
action can fetch and refresh the GitHub OIDC identity token itself, and
expose the same federation inputs as the outer action. Switch the
base-action test workflows from the anthropic_api_key secret to the
federation repo variables and grant them id-token: write.

* Verify MCP test tool invocation instead of init connection status

MCP servers can connect asynchronously, so the init event may report a
server as pending. Check that the server is registered at init, then
assert the test tool was actually called and returned its response.
Also pass the MCP config through claude_args --mcp-config, replacing the
removed mcp_config input.

.github/workflows/test-custom-executables.yml

@@ -5,6 +5,13 @@ on:
   workflow_dispatch:
   workflow_call:
 
+# The Claude API is authenticated via workload identity federation: id-token
+# lets the action mint the GitHub OIDC token it exchanges for a short-lived
+# access token. See docs/setup.md.
+permissions:
+  contents: read
+  id-token: write
+
 jobs:
   test-custom-executables:
     runs-on: ubuntu-latest
@@ -47,7 +54,9 @@ jobs:
         with:
           prompt: |
             List the files in the current directory starting with "package"
-          anthropic_api_key: ${{ secrets.ANTHROPIC_API_KEY }}
+          anthropic_federation_rule_id: ${{ vars.ANTHROPIC_FEDERATION_RULE_ID }}
+          anthropic_organization_id: ${{ vars.ANTHROPIC_ORGANIZATION_ID }}
+          anthropic_service_account_id: ${{ vars.ANTHROPIC_SERVICE_ACCOUNT_ID }}
           path_to_claude_code_executable: /home/runner/.local/bin/claude
           path_to_bun_executable: /home/runner/.bun/bin/bun
           allowed_tools: "LS,Read"