feat(eval): store eval files outside a read-only agents_dir

[Anusha0501]

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Link to Issue or Description of Change

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

• Closes: Feature Request: Support Custom Evalset Storage Directory via Environment Variable #3887

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

Problem:
Local eval sets and results are written under agents_dir. In Kubernetes and other container deployments that directory is often read-only, so adk web / adk eval fail with PermissionError. --eval_storage_uri only accepted gs://, which is more infrastructure than many environments need.

Solution:
Resolve eval storage with this precedence: explicit --eval_storage_uri / eval_storage_uri, then ADK_EVAL_STORAGE_URI, then ADK_EVAL_STORAGE_DIR, then agents_dir. file:// URIs (and ADK_EVAL_STORAGE_DIR) use the existing local eval managers and create the directory if needed. GCS behavior is unchanged.

export ADK_EVAL_STORAGE_DIR=/tmp/adk_evals
# or
export ADK_EVAL_STORAGE_URI=file:///tmp/adk_evals
adk web /app/agents

app = get_fast_api_app(
    agents_dir="/app/agents",
    eval_storage_uri="file:///tmp/adk_evals",
    web=True,
)

Testing Plan

Unit Tests:

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

PYTHONPATH=src python -m pytest \
  tests/unittests/cli/utils/test_evals.py \
  tests/unittests/cli/utils/test_cli_eval.py \
  -q -k "not test_create_gcs_eval_managers_from_uri_success"

41 passed, 1 deselected

The deselected case needs google-cloud-storage (existing GCS manager test). New coverage includes URI/env precedence, file:// path decoding, directory creation, ~ expansion, and get_eval_sets_manager / get_fast_api_app wiring.

Manual End-to-End (E2E) Tests:

1. Point agents_dir at a read-only tree (or chmod a-w).
2. export ADK_EVAL_STORAGE_DIR=/tmp/adk_evals
3. Create an eval set via adk eval_set create or the web eval UI.
4. Confirm the .evalset.json is written under /tmp/adk_evals/<app_name>/ and not under the agent directory.

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

Does not add a new eval_storage_dir= kwarg on get_fast_api_app; eval_storage_uri="file://..." and the env vars cover the same use case with one resolution path.