fix(litellm): forward GenerateContentConfig.seed to LiteLLM

[chelsealong]

Summary

GenerateContentConfig(seed=...) was silently dropped by LiteLlm — the seed
stayed in the request config but never reached the parameters passed to
LiteLLM.

The cause is in _get_completion_inputs (src/google/adk/models/lite_llm.py):
generation parameters are extracted by iterating over a fixed allowlist of keys
(temperature, max_output_tokens, top_p, top_k, stop_sequences,
presence_penalty, frequency_penalty), and seed was not in that list.
LiteLLM supports seed natively (see
https://docs.litellm.ai/docs/completion/input), so the one-line fix adds it to
the forwarded parameters.

Closes #6513

Testing Plan

Extended the existing test_get_completion_inputs_generation_params unit test
to set seed=42 and assert generation_params["seed"] == 42.

Proof the test guards the fix (stash source change, run test, restore):

• Without the fix: assert generation_params["seed"] == 42 →
  KeyError: 'seed' (1 failed)
• With the fix: 1 passed

Full suite for the touched module:

$ pytest tests/unittests/models/test_litellm.py -q
356 passed, 5 warnings

Lint: pyink --check, isort --check-only, and ruff check all pass on the
changed files.

Note

This PR was prepared with AI assistance (Claude). The fix, the test, and the
test evidence above were reviewed for correctness before submitting.

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[chelsealong]

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[chelsealong]

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