fix(cli): lazy-import GCS deps so adk web works without google-cloud-storage

[mangod12]

Summary

• Lazy-import GCS modules in google.adk.cli.utils.evals so DevServer imports successfully without google-cloud-storage installed
• Add python-multipart to core dependencies since it's required for Builder UI endpoints in adk web

Root cause

evals.py unconditionally imports GcsEvalSetResultsManager and GcsEvalSetsManager at module level. These modules import google.cloud.storage, which is only declared under [gcp]/[all]/[test] extras. Since dev_server.py imports evals, a fresh pip install google-adk causes DevServer to fail to import, silently falling back to ApiServer — leaving the dev UI at /dev-ui/ unreachable (404).

Changes

• src/google/adk/cli/utils/evals.py — moved GCS imports behind TYPE_CHECKING for annotations and lazy-import inside create_gcs_eval_managers_from_uri() where they're actually needed
• pyproject.toml — added python-multipart>=0.0.9 to core dependencies
• tests/unittests/cli/utils/test_evals.py — updated mock paths and added regression test verifying GCS classes are not in module namespace at runtime

Test plan

• All 3 tests in test_evals.py pass (existing + new regression test)
• from google.adk.cli.dev_server import DevServer succeeds
• Pre-commit hooks pass on all changed files
• Consistent with existing lazy-import pattern used in fast_api.py (lines 519-531) for DevServer itself

pytest tests/unittests/cli/utils/test_evals.py -v
3 passed in 9.65s

Fixes #5787

🤖 Generated with Claude Code

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[mangod12]

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