fix(cli): preserve Docker dependency cache

[mikemikimike]

Issue

Closes #6923.

Problem

The Dockerfile generated by adk deploy copied the complete agent source before installing the agent's requirements.txt. As a result, a source-only change invalidated the dependency layer and caused dependencies to be downloaded again.

Changes

• Copy an agent's requirements.txt into its own Docker layer when the file is present.
• Install agent dependencies before copying the remaining agent source.
• Apply the ordering consistently to Cloud Run, Agent Engine, and GKE while preserving the existing no-requirements behavior.
• Add a regression assertion for the generated Dockerfile instruction order.

Testing

• .venv/Scripts/python.exe -m pytest tests/unittests/cli/utils/test_cli_deploy.py::test_to_agent_engine_happy_path tests/unittests/cli/utils/test_cli_deploy.py::test_to_gke_happy_path tests/unittests/cli/utils/test_cli_deploy_to_cloud_run.py::test_to_cloud_run_happy_path -q — 8 passed.
• uvx --from pyink==25.12.0 pyink --check src/google/adk/cli/cli_deploy.py tests/unittests/cli/utils/test_cli_deploy_to_cloud_run.py — passed.
• python -m ruff check src/google/adk/cli/cli_deploy.py tests/unittests/cli/utils/test_cli_deploy_to_cloud_run.py — passed.
• python -m compileall -q src/google/adk/cli/cli_deploy.py — passed.
• git diff --check — passed.
• .venv/Scripts/python.exe -m pytest tests/unittests/cli/utils/test_cli_deploy.py tests/unittests/cli/utils/test_cli_deploy_to_cloud_run.py -q — 76 passed, with the 10 known failures described below.
• mypy --config-file pyproject.toml src/google/adk/cli/cli_deploy.py --follow-imports=skip --ignore-missing-imports — 4 existing errors outside the changed lines.

Compatibility and known limitations

When no requirements.txt exists, no requirements copy or install step is emitted. The two affected deployment test files report 76 passed and 10 unrelated Windows failures because their tests mock shutil.rmtree with a one-argument lambda while Python 3.12 calls it with the onexc keyword; those tests were not modified. Mypy also reports four existing errors at lines 523, 1455, 1467, and 1702. The targeted deployment tests pass; no cloud credentials or external services are required.