fix(cli): ignore generated ADK runtime data

[uuzzrm]

Summary - Extend adk create to ignore the generated .adk/ runtime directory. - Preserve existing .gitignore entries and avoid duplicating either generated entry. - Add regression coverage for new, existing, newline-terminated, and partially populated .gitignore files. Fixes #6647. ## Testing - pytest tests/unittests/cli/utils/test_cli_create.py -q (31 passed) - ruff check src/google/adk/cli/cli_create.py tests/unittests/cli/utils/test_cli_create.py - pyink --check src/google/adk/cli/cli_create.py tests/unittests/cli/utils/test_cli_create.py - git diff --check The tests run without network access, model access, or credentials. ## Notes This change was prepared with Codex assistance and reviewed against the repository's contribution guidelines. The implementation is intentionally limited to the CLI scaffold's generated-file ignore list and its unit tests.

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AI assistance (Codex) was used for repository research, implementation, and test drafting. The diff and validation results are documented here for maintainer review.

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[uuzzrm]

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[uuzzrm]

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[uuzzrm]

The Google CLA check is now passing, and all current checks are green. The PR is ready for review. Thanks!

[uuzzrm]

Closing this duplicate PR because #6649 already covers the same fix for #6647. The existing PR is the appropriate place for maintainer review.

[uuzzrm]

Closing this duplicate PR; #6649 already covers #6647 and is the appropriate place for maintainer review.