test(evaluation): add unit tests for _eval_set_results_manager_utils

[Koushik-Salammagari]

Link to Issue or Description of Change

Description of the change (no existing issue):

Problem:
The google.adk.evaluation._eval_set_results_manager_utils module had no
dedicated unit-test coverage. It contains non-trivial logic that is easy to
regress, most notably parse_eval_set_result_json, which has a
backward-compatibility fallback for legacy double-encoded result files (where
the outer JSON is a string containing the inner JSON object).

Solution:
Add a focused unit-test module that covers all three functions and their edge
cases. No production code is changed — this is a test-only PR that locks in the
current behavior.

Coverage added:

• _sanitize_eval_set_result_name — slash replacement, multiple slashes, and
  no-op cases.
• create_eval_set_result — {app_name}_{eval_set_id}_{timestamp} id
  encoding, that the timestamp matches creation_timestamp, name sanitization,
  and empty eval_case_results.
• parse_eval_set_result_json — standard JSON string, bytes input,
  camelCase-aliased JSON, the legacy double-encoded format, and error paths for
  a JSON object missing required fields and for non-JSON input.

Testing Plan

Unit Tests:

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

$ pytest tests/unittests/evaluation/test__eval_set_results_manager_utils.py -v
...
14 passed in 1.08s

Manual End-to-End (E2E) Tests:

Not applicable — this is a test-only change with no runtime/user-facing impact.

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.

[rohityan]

Hi @Koushik-Salammagari , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Your PR has been received by the team and is currently under review. We will provide feedback as soon as we have an update to share.

[rohityan]

Hi @DeanChensj , can you please review this.