test(scheduler): add unit tests for Healthy, LastTickAt, ComputeNextRun, panic recovery

[HongmingWang-Rabbit]

Summary

Adds platform/internal/scheduler/scheduler_test.go — the only previously untested security-critical package from PR #90.

• 8 new tests covering every testable code path in the scheduler
• 4 minimal production changes to scheduler.go required to make the code testable (all additive, no behaviour change for existing callers)
• 1 bug fix: ComputeNextRun now returns an error on invalid timezone instead of silently falling back to UTC

Tests added

Test	What it covers
TestLastTickAt_zero	LastTickAt() returns zero time.Time before the first tick
TestHealthy_beforeStart	Healthy() is false on a fresh scheduler (zero lastTickAt)
TestHealthy_freshTick	Healthy() is true when lastTickAt == now
TestHealthy_stale	Healthy() is false when lastTickAt is 3×pollInterval ago (beyond 2× threshold)
TestComputeNextRun_valid	"0 * * * *" / UTC returns a future top-of-hour time, no error
TestComputeNextRun_invalid	Unparseable cron expression returns non-nil error
TestComputeNextRun_invalidTimezone	Unrecognised IANA timezone returns non-nil error
TestPanicRecovery	panicProxy panics inside ProxyA2ARequest; scheduler goroutine recovers and Healthy() remains true — an unrecovered panic would crash the test binary, so the test completing is proof

Production changes to scheduler.go

Change	Rationale
Added mu, lastTickAt, tickInterval fields	Required for LastTickAt(), Healthy(), and test-speed control
Added LastTickAt() / Healthy() methods	The observable surface the tests exercise
tick() records lastTickAt after wg.Wait()	Single mutex write per tick; zero hot-path cost
fireSchedule() deferred recover()	Without this, a panicking proxy crashes the goroutine pool; the test binary itself would crash without it
ComputeNextRun returns error on bad timezone	Previously silently fell back to UTC; handler already validates tz before calling, so no callers break

Test output

=== RUN   TestLastTickAt_zero          --- PASS (0.00s)
=== RUN   TestHealthy_beforeStart      --- PASS (0.00s)
=== RUN   TestHealthy_freshTick        --- PASS (0.00s)
=== RUN   TestHealthy_stale            --- PASS (0.00s)
=== RUN   TestComputeNextRun_valid     --- PASS (0.00s)
=== RUN   TestComputeNextRun_invalid   --- PASS (0.00s)
=== RUN   TestComputeNextRun_invalidTimezone --- PASS (0.00s)
=== RUN   TestPanicRecovery            --- PASS (0.00s)
ok   github.com/Molecule-AI/molecule-monorepo/platform/internal/scheduler   0.003s

Full suite: all packages pass, zero failures.

Test plan

• go test ./internal/scheduler/... -v — 8/8 pass
• go test ./... — full suite passes, no regressions

🤖 Generated with Claude Code

[HongmingWang-Rabbit]

Superseded by #119 (cherry-picked onto a clean base from main — the original branch was pre-#95/#99/#106 and showed -965 spurious deletions). The commit 9a66a4c is included verbatim in #119.