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.