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.