fix: raise clear ImportError when VertexAiSearchTool bypass needs google-adk[gcp]

[chelsealong]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Related: bypass_multi_tools_limit on VertexAiSearchTool is a leaky abstraction: unexpected dependencies, silent shift from inbuilt RAG to tool calling, and prompt fragility #7100

2. Or, if no issue exists, describe the change:

Problem:
Issue #7100 reports several problems with VertexAiSearchTool(bypass_multi_tools_limit=True). This PR addresses one specific, well-scoped part of it: point 4 in the issue's "Suggested Improvements" ("Fail Fast with Clear Dependency Errors"). Setting bypass_multi_tools_limit=True silently swaps VertexAiSearchTool for DiscoveryEngineSearchTool, which requires the google-cloud-discoveryengine package (the google-adk[gcp] extra). When that package isn't installed, the failure previously surfaced as a bare ModuleNotFoundError from deep inside discovery_engine_search_tool.py, with no indication of what to install.

The other points raised in the issue (allowing a custom tool name/description, aliasing/fuzzy tool-name resolution, and whether to deprecate bypass_multi_tools_limit in favor of a different pattern) are API-shape/design decisions for maintainers to weigh in on, so this PR does not attempt them.

Solution:
Wrap the lazy DiscoveryEngineSearchTool import in _convert_tool_union_to_tools (src/google/adk/agents/llm_agent.py) in a try/except ImportError, re-raising with a message that names the missing dependency and the exact install command (pip install google-adk[gcp]).

Testing Plan

Unit Tests:

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

Added test_handle_vais_with_other_tools_missing_gcp_extra to tests/unittests/agents/test_llm_agent_fields.py, which simulates the missing dependency via sys.modules patching and asserts the new, actionable ImportError message is raised.

Confirmed the new test fails without the fix (reverting llm_agent.py to HEAD~1 and re-running):

$ python -m pytest tests/unittests/agents/test_llm_agent_fields.py -k vais -q
...
FAILED ...test_handle_vais_with_other_tools_missing_gcp_extra - AssertionError: Regex pattern did not match.
  Expected regex: 'google-adk\\[gcp\\]'
  Actual message: 'import of google.adk.tools.discovery_engine_search_tool halted; None in sys.modules'
1 failed, 5 passed, 89 deselected in 5.55s

With the fix applied:

$ python -m pytest tests/unittests/agents/test_llm_agent_fields.py -k vais -q
......                                                                   [100%]
6 passed, 89 deselected in 2.49s

Full unit test suite, unaffected:

$ python -m pytest tests/unittests -q
14670 passed, 82 skipped, 27 xfailed, 2 xpassed, 2009 warnings, 28 subtests passed in 405.19s

pyink --check on the changed files reports no formatting issues.

Manual End-to-End (E2E) Tests:

Not applicable — this is a small, targeted error-message change exercised entirely by the added unit test; no live GCP/Discovery Engine access is needed to observe the behavior.

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

This PR intentionally does not attempt the other suggestions in #7100 (custom tool naming/description, tool-name alias resolution, deprecating bypass_multi_tools_limit), since those involve API-shape decisions that should be made by maintainers rather than assumed by a contributor.

🤖 Generated with Claude Code

[adk-bot]

Thank you @chelsealong for your contribution! 🎉

Your changes have been successfully imported and merged via Copybara in commit 4e73b7d.

Closing this PR as the changes are now in the main branch.