fix(cli): require OIDC audience when trigger endpoints are enabled

[Tednoob17]

Link to Issue or Description of Change

Problem:
The /trigger/pubsub and /trigger/eventarc endpoints do not authenticate requests when trigger_sources is enabled but neither trigger_oidc_audience nor trigger_auth_verifier is configured. This leaves trigger endpoints unauthenticated.

Solution:
Require trigger_oidc_audience or trigger_auth_verifier whenever trigger_sources is set, both at server startup and at deploy time.

Testing Plan

Unit Tests:

• Added src/google/adk/cli/test_trigger_auth.py
• All unit tests pass locally

pytest results: 3 passed

Manual End-to-End (E2E) Tests:
N/A for this change.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.