Compose flaky attribute with --flaky_test_attempts

[alexeagle]

Summary

Bazel's flaky BUILD attribute and --flaky_test_attempts flag are disconnected: a global numeric flag applies the same retry count to every target (ignoring flaky=1), while the attribute only affects behavior when the flag is left at default and cannot express degrees of flakiness. This makes it hard to declare per-target retry policy in BUILD files at scale.

Upstream tracking issue: bazelbuild#30108

This change makes flaky an integer (additional retry count when the flag is default) and clarifies how it composes with --flaky_test_attempts.

Semantic changes

--flaky_test_attempts	flaky	Before	After
default (unset)	0 / False	1 attempt	1 attempt
default	1 / True	3 attempts	2 attempts (1 + 1)
default	2	invalid (analysis error)	3 attempts (1 + 2)
default	N	invalid	1 + N attempts
numeric (e.g. 3)	0	3 attempts	3 attempts
numeric (e.g. 3)	1	3 attempts	3 attempts (attribute ignored)
numeric (e.g. 3)	2	3 attempts	3 attempts (attribute ignored)

flaky=True / flaky=False continue to work (True → 1, False → 0).

Breaking changes / migration

Use case	Impact	Migration
Targets with flaky=1 relying on 3 attempts under default flag	Now get 2 attempts	Set flaky=2 to restore 3 attempts
CI using --flaky_test_attempts=N expecting only flaky=1 targets to retry	Unchanged	—
Per-target retry counts scattered across a monorepo	Previously required long regex@N lists on the flag	Set flaky=N per target when flag is default
flaky=2 or higher in BUILD files	Previously failed analysis	Now valid

Test plan

• TestStrategyTest — flag override vs default composition
• TestActionBuilderTest — flaky=True, integer values, negative rejection
• TypeTest — FLAKY_TEST_RETRIES conversion and tagging