Protect testcase task-log downloads

[M0nd0R]

This PR is a clean replacement for #5214 with only the task-log fix and its focused tests.

It adds testcase access checks before returning task logs and quotes task log filter values before building the Cloud Logging query so user-controlled task parameters cannot alter the intended filter constraints.

Verification:

• targeted smoke check: TaskLogHandler now calls access.check_access_and_get_testcase(123) before fetching logs
• targeted smoke check: injected filter content remains inside the quoted task_id string literal
• local pytest selection reaches the new tests, but full execution is blocked here because this environment does not have gcloud for the datastore emulator

[decoNR]

Please fix lint errors (ref: Run basic tests / build (pull_request)).

[decoNR]

/gcbrun

[M0nd0R]

Addressed the review about the unnecessary cast. The handler now passes the raw testcase_id string into check_access_and_get_testcase, relies on data_handler.get_testcase_by_id for validation, and uses the returned testcase key ID for the log lookup. I also updated TaskLogHandlerTest to cover that flow explicitly.

[decoNR]

/gcbrun

[decoNR]

LGTM, thanks! We will probably deploy it in the next release

[ViniciustCosta]

lgtm