fix(cli): install agent deps before copying source in deploy Dockerfiles

[Anusha0501]

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Link to Issue or Description of Change

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

• Closes: Dockerfile should first copy requirements.txt and install dependencies before copying agent code #6923

2. Or, if no issue exists, describe the change:

Problem:
adk deploy Dockerfiles copy the entire agent tree before pip install -r requirements.txt. Any source-only change invalidates the dependency layer and re-downloads packages.

Solution:
Copy requirements.txt into its own layer and install it before copying the rest of the agent. Cloud Run, Agent Engine, and GKE all share _DOCKERFILE_TEMPLATE, so the order is applied in one place. Agents without requirements.txt still emit the existing no-op comment.

Testing Plan

Unit Tests:

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

PYTHONPATH=src pytest tests/unittests/cli/utils/test_cli_deploy.py::test_agent_deps_install_layer_copies_requirements_before_pip tests/unittests/cli/utils/test_cli_deploy.py::test_agent_deps_install_layer_without_requirements tests/unittests/cli/utils/test_cli_deploy.py::test_to_gke_happy_path tests/unittests/cli/utils/test_cli_deploy_to_cloud_run.py

24 passed

Asserts that COPY requirements.txt comes before RUN pip install and that both come before the full agent COPY.

Manual End-to-End (E2E) Tests:

Generate a Cloud Run Dockerfile with adk deploy cloud_run for an agent that has requirements.txt and confirm the instruction order is: copy requirements, pip install, copy agent source.

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.

Additional context

#6928 attempted the same cache-order fix and was closed unmerged. This reapplies it on current _DOCKERFILE_TEMPLATE for all three deploy targets.