Deny local credential-printing gcloud commands by default

[fsabiu]

Title

Deny local credential-printing gcloud commands by default

Body

This change adds local credential extraction commands to the default denylist used by gcloud-mcp.

The MCP tool returns stdout from allowed gcloud commands to the client/model. Commands such as auth print-access-token, auth application-default print-access-token, auth print-identity-token, and config config-helper can directly print transferable local credential material, so they should be denied by default in the same way that SSH, interactive, and meta command groups are denied by default.

The patch also adds regression coverage that:

• verifies the default denylist contains the credential-printing commands;
• verifies a denylisted credential-printing command does not invoke gcloud;
• verifies fake token stdout is not returned when the command is denied.

Tests run:

npx vitest run src/index.test.ts src/tools/run_gcloud_command.test.ts src/denylist.test.ts --reporter=verbose --coverage=false
npm run build
npx prettier --check src/index.ts src/index.test.ts src/tools/run_gcloud_command.test.ts
npx eslint src/index.ts src/index.test.ts src/tools/run_gcloud_command.test.ts --max-warnings 0