Add stable,flaky syntax to --flaky_test_attempts

[alexeagle]

Summary

• Extend --flaky_test_attempts to accept two comma-separated integers (<stable>,<flaky>) so CI can retry only flaky-marked targets without changing BUILD files.
• Preserve backward compatibility: a bare integer (e.g. 3) still applies to all tests; default still means stable=1 / flaky=3.
• Per-target regex@N overrides remain supported with a single integer only (regex@1,3 is rejected).

Upstream PR: bazelbuild#30638
Tracking issue: bazelbuild#30108

Semantics

Flag value	flaky=False	flaky=True
(unset) / default	1	3
3	3	3
1,3	1	3
1,5	1	5
//pkg/@3 (matched targets)	3	3

If upstream never merges, are we stuck carrying this patch forever?

No — but we would need to keep carrying it as long as we want --flaky_test_attempts=1,3 in .bazelrc.

This patch is small and isolated (~300 lines, mostly tests; core logic is FlakyTestAttempts + converter + TestStrategy lookup). It does not fork broad Bazel subsystems, and it rebases cleanly onto upstream releases (the upstream PR is the same diff against master).

If upstream merges: drop the fork patch on the next Bazel version bump / rebase from bazelbuild/bazel. No migration needed — behavior and flag syntax are identical.

If upstream does not merge: we have three options, none of which block upgrading Bazel:

1. Keep cherry-picking the patch on each Bazel bump. Cost is low: touch-conflict surface is 3–4 files, and the change is backward-compatible so it does not perturb unrelated behavior.
2. Revert the patch and change .bazelrc back to stock semantics. --flaky_test_attempts=3 still works on vanilla Bazel (retries all tests). We lose the stable/flaky split until we find another approach (e.g. integer flaky attribute, or regex@N lists).
3. Stop using the new syntax without reverting the patch. Unused syntax in the binary is harmless; if CI never passes 1,3, behavior matches stock Bazel.

The patch is a capability opt-in, not a lock-in. Stock Bazel continues to work; we only depend on this fork delta for the specific .bazelrc value 1,3 (or other <stable>,<flaky> pairs).

Test plan

• FlakyTestAttemptsTest — parsing and validation
• TestAttemptsConverterTest — global, regex, and rejection of regex@1,3
• test_flaky_test_attempts_stable_flaky_split shell test — verifies 1,2 split and bare integer backward compat
• Full CI