fix(cli): reload changed agent correctly

[gioboa]

Close #4956

• Please ensure you have read the contribution guide before creating a pull request.

Problem:

When agent hot reload detected a modified file, it reloaded the agent currently open in ADK Web instead of the agent that owned the modified file.

Solution:

Pass the agents directory to AgentChangeEventHandler and derive the affected agent name from the modified file path. This keeps the fix local to hot reload and avoids changing any supported public API or CLI option.

Testing Plan

Please describe the tests that you ran to verify your changes. This is required
for all PRs that are not small documentation or typo fixes.

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

tests/unittests/cli/utils/test_agent_change_handler.py
9 passed

tests/unittests
11879 passed, 92 skipped, 25 xfailed, 1 xpassed, 19 subtests passed

Manual End-to-End (E2E) Tests:

Create a two-agent setup, read agent B’s metadata via GET /apps/agent_b/app-info, modify its description, then verify the endpoint returns the updated metadata.

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.

Additional context

No documentation changes are required.