Preserve due-schedule timestamp precision

[rmcdaniel]

Summary

Refs #505. Preserve the WorkflowSchedule model's microsecond serialization in the due-schedule predicate, instead of letting the connection format a DateTime binding at whole-second precision. No API, schema, cron or overlap-policy change.

The regression persists and reloads the deadline, freezes time at exact whole-second/fractional boundaries, checks original occurrence identity, forbids early dispatch and verifies that a repeated tick does not start twice. Existing fairness checks are unchanged.

Validation

• Before the fix: three due boundary cases fail on SQLite; the future case passes.
• After the fix: all seven tick cases pass, 36 assertions, PHP 8.4 / Laravel 13 / SQLite.
• ECS applied to the two changed files; diff reviewed.
• Broader schedule/degraded-mode/command tests and static analysis are running. Repository CI will provide normal MySQL and release-gate checks.

Release Follow-Through

Keep #505 open until publication and affected-consumer qualification. Inspect Server's locked Workflow/image contents and embedded consumers; SDK/CLI protocol and packages are unchanged. Run the published schedule cases on SQLite/MySQL/PostgreSQL after publication; record downstream rebuild/release decisions rather than assuming a library tag changes existing images.

[rmcdaniel]

Local verification is complete for SQLite: 89 schedule, degraded-mode and command cases / 452 assertions pass, and full phpstan analyse src tests reports no errors. ECS passes for the changed files. Before the fix, the three due boundary cases failed; after it all seven tick cases pass, including namespace fairness and duplicate-tick prevention.

The first broad local command used a synchronous queue, which is deliberately unsupported by four degraded-mode cases. Rerunning with the asynchronous database queue resolved those harness errors without changing tests or product code. Existing CI remains the public MySQL gate; PostgreSQL qualification and published-artifact follow-through remain outstanding.