fix(workflow): treat RetryConfig(max_attempts=0) as no retries

[Osamaali313]

Fixes #6293

Problem

RetryConfig.max_attempts is documented as "Maximum number of attempts, including the original request. If 0 or 1, it means no retries. If not specified, default to 5." and defaults to None. But _should_retry_node resolved the default with a falsy-coalescing or:

# src/google/adk/workflow/utils/_retry_utils.py:35
max_attempts = retry_config.max_attempts or 5

0 is falsy, so RetryConfig(max_attempts=0) — documented as "no retries" — is silently replaced with 5, running 4 retries instead of none. max_attempts=1 happens to work (1 or 5 == 1); only the documented 0 case is broken. The intended "unset" sentinel is None, which must still map to 5.

Workflow and RetryConfig are public API (both in __all__, no @experimental gate), and this drives every workflow node's retry loop via _NodeRunner._attempt_retry.

Fix

max_attempts = (
    retry_config.max_attempts if retry_config.max_attempts is not None else 5
)

Tests

Added TestShouldRetryNode to tests/unittests/workflow/utils/test_retry_utils.py (the file previously covered only _get_retry_delay), asserting:

• max_attempts of 0 and 1 disable retries
• max_attempts=3 retries until the limit
• unset (None) defaults to 5

Before the fix, the max_attempts=0 case fails; after, it passes. The full tests/unittests/workflow/ suite passes (619 passed, no regressions).

[Osamaali313]

Pushed a small formatting commit — pyink (line-length 80) wanted the long _should_retry_node assertions in test_retry_utils.py wrapped; I ran pyink with the repo config and it's now clean (pyink --check passes, tests still green: 10 passed).

Heads up on the other red check: the Mypy failure isn't from this PR's diff — the "new" errors it lists are all in src/google/adk/models/lite_llm.py (_finish_reason_to_error_message arg-type), which this PR doesn't touch. Both baseline and PR report 3375 errors; it looks like the baseline-diff surfaced a pre-existing lite_llm.py typing issue after the main merge. Happy to help if you'd like, but it's outside this change.

[rohityan]

Hi @Osamaali313 , 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 @wyf7107 , can you please review this. LGTM.