Add cap for parked tasks in send scheduler

[stubbsta]

Background

Budget-parked tasks are exempt from the reaper (firstAdmittedTime is never set) and taskCache has no cap, so sustained over-budget sends grow the backlog without bound and deliver arbitrarily late. Flagged as an accepted review finding on #4062.

Details

Expire never-admitted parked tasks once messageAge() exceeds a configurable maximum, emitting MessageErrorEvent so the application gets explicit feedback. Additionally enforce a configurable hard cap on taskCache, rejecting new sends with backpressure feedback when full.

Acceptance criteria

• A parked task past the age limit is dropped with a MessageErrorEvent; taskCache never exceeds the cap, and sends beyond it are rejected with feedback rather than silently queued.
• Tasks admitted before the age limit are unaffected — the existing park→release-on-epoch-roll behavior stays green in the scheduler tests.