fix: silence false-positive --explain hints and preview selected FQNs at lifecycle level

[vedanthvdev]

Summary

Batch-3 of the post-v1.9.17 sanity-test fixes. Two user-visible
changes, one latent-bug cleanup that falls out of the refactor.

1. --explain hint gating

Running v1.9.17 across nine representative MR shapes on the
security-service pilot surfaced the Hint: outOfScopeTestDirs is configured but no file in the diff matched any configured pattern
line firing on five of them — including a markdown-only MR where
nothing a source-tree matcher could have bitten was even in the diff.
That 5-of-9 false-positive rate trains reviewers to ignore the hint,
which is exactly the anti-pattern it was added in v1.9.14 to prevent.

The hint is now gated on Situation.DISCOVERY_SUCCESS or
Situation.DISCOVERY_EMPTY — the only two branches where a
correctly-configured out-of-scope pattern could have changed the
outcome. The other four situations suppress:

Situation	Why it can't be silent
EMPTY_DIFF	No files to match
ALL_FILES_IGNORED	Ignore wins before out-of-scope ever looks
ALL_FILES_OUT_OF_SCOPE	Bucket is non-empty (guard was already there, gate makes it explicit)
UNMAPPED_FILE	File missed every pattern, including out-of-scope

Each negative case has an explicit test so the gate can't drift.

2. Lifecycle FQN preview for SELECTED dispatches

Pre-v1.9.18 the task printed only the module summary at lifecycle
level (application:test (47 test classes)) and demoted every FQN to
info. A reviewer scrolling the default CI log could see the dispatch
size but not which tests ran without either re-running with
--info (slow) or opening the JUnit report after the fact.

renderLifecycleDispatchPreview is a new pure helper that emits the
first 5 FQNs per module at lifecycle level with a
… and N more (use --info for full list) tail on larger dispatches.
The 5-class cap keeps total lifecycle output well under the 4 MiB
GitHub Actions step cap that forced the info-level demotion in the
first place. Info-level per-FQN logging is retained for --info
users.

Example lifecycle output (17-class dispatch):

Running 17 affected test classes across 1 module(s):
  application:test (17 test classes)
    com.example.AuthenticatorTest
    com.example.TokenValidatorTest
    com.example.SessionStoreTest
    com.example.PasswordResetTest
    com.example.MfaChallengeTest
    … and 12 more (use --info for full list)

3. Header arithmetic fix + silent-fallback closure

Hoisting FQN validation out of the per-module log loop to feed the
lifecycle preview also fixed two latent issues:

• Header count inconsistency. The Running N affected test classes
  header used the pre-validation count while per-module counts were
  post-validation. Skipped FQNs now surface in a
  (K malformed FQN skipped — see WARN above) suffix.
• Silent full-module fallback. A module whose entire discovered
  FQN set failed isValidFqn used to dispatch its bare taskPath
  with no --tests filter, silently escalating to a full module
  suite — the exact safety regression isValidFqn was added to
  prevent. Modules with zero valid FQNs are now dropped from dispatch,
  and a cross-all-modules zero-survivors dispatch throws
  GradleException instead of invoking Gradle with an empty task
  list. CHANGELOG calls both behavior deltas out for adopters.

Test plan

• ./gradlew check — green
• 5 new negative hint-gate tests in AffectedTestTaskExplainFormatTest (one per suppressed situation, plus an explicit ALL_FILES_OUT_OF_SCOPE pin)
• 1 new positive hint-gate test covering the DISCOVERY_EMPTY full-suite escalation path
• 5 new dispatch-preview tests (empty, singleton, exactly-at-limit, oversized, 200-FQN stress) covering format, pluralization, ordering, and tail
• Sanity-retested on security-service against 9 MR shapes; hint now silent on the 4 previously-false-positive cases, still fires on the correctly-silent misconfig scenario, preview renders with the expected tail
• Code-reviewer sweep (must-fix: none; should-fix: added empty-validation GradleException guard + CHANGELOG bullet for the silent-fallback behavior change; nits: FQN pluralization paper-cut fixed)