fix(agents): reject candidate_count greater than one

[claxman]

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Link to Issue or Description of Change

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

• Closes: candidate_count > 1 is ignored by LiteLLM and discarded by other adapters #6518
• Related: fix: reject multiple response candidates #6519

Problem:
LlmAgent(name='x', generate_content_config=GenerateContentConfig(candidate_count=2)) constructs with candidate_count 2. Extra candidates are dropped at LlmResponse.create, which keeps candidates[0]. 25f5214 (from #6519) logs the drop in the adapters but construction stays silent.

Issue script on b0180620, PYTHONPATH=src:

LiteLLM request params: None
LiteLLM returned texts: ['first', 'second']
ADK converted texts: ['first']

Solution:
Reject candidate_count > 1 in LlmAgent.validate_generate_content_config, the validator that already rejects tools, system_instruction, response_schema, and http_options.base_url. The error names generate_content_config= and says extras are discarded. candidate_count=1 stays allowed.

Adapters are left at 25f5214. #6519 put the check there and the import kept only the log. Mapping candidate_count to n and returning extras is out: LlmResponse is single-candidate. svfat filed the issue and the first raise.

Testing Plan

Unit Tests:

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

New tests: test_validate_generate_content_config_candidate_count_throw and test_validate_generate_content_config_candidate_count_one_allowed in tests/unittests/agents/test_llm_agent_fields.py, plus a case in tests/unittests/agents/test_llm_agent_error_messages.py::TestValidateGenerateContentConfigErrors.

PYTHONPATH=src .venv/bin/python -m pytest -q \
  tests/unittests/agents/test_llm_agent_fields.py::test_validate_generate_content_config_candidate_count_throw \
  tests/unittests/agents/test_llm_agent_fields.py::test_validate_generate_content_config_candidate_count_one_allowed \
  tests/unittests/agents/test_llm_agent_error_messages.py::TestValidateGenerateContentConfigErrors

6 passed.

Removed the validator block and reran the two throw tests. Both failed with DID NOT RAISE ValueError. Restored the block.

PYTHONPATH=src .venv/bin/python -m pytest -q tests/unittests -n auto

13981 passed, 84 skipped, 27 xfailed, 2 xpassed in 137s.

Manual End-to-End (E2E) Tests:

No live provider key. After the change, LlmAgent(name='x', generate_content_config=types.GenerateContentConfig(candidate_count=2)) raises a pydantic ValidationError whose value error is candidate_count must be 1 or unset. LlmResponse keeps one candidate, so extra values are requested then discarded. Pass generate_content_config=types.GenerateContentConfig() without candidate_count, or set candidate_count=1.

Additional context

Asked xuanyang15 on #6518 whether construct-time raise is wanted after 25f5214. #6518 (comment)

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.

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