Schedule HTTP/2 re-enable on NetVConnection owner thread

[saimayithri]

Refs: #13358

Summary

Http2CommonSession::state_complete_frame_read() schedules
HTTP2_SESSION_EVENT_REENABLE on the thread currently holding the session
mutex.

That thread can differ from the underlying NetVConnection owner thread.
When the re-enable handler later processes frames or releases streams, it can
reach UnixNetVConnection queue operations from the wrong thread and trigger
the NetHandler lock assertion described in #13358.

This change schedules the re-enable event on the session NetVConnection's
owner thread instead.

Changes

At both HTTP/2 re-enable scheduling sites:

• Keeps the existing continuation, delay, event type, and VIO unchanged.
• Uses get_netvc()->thread as the scheduling target instead of
  get_mutex()->thread_holding.
• Asserts that the session has an associated NetVConnection before
  scheduling.

Validation

• Built successfully with:
  cmake --build build-dev -j4

• Ran:
  ctest --test-dir build-dev -j4

Scope

This PR addresses the HTTP/2 re-enable path from #13358 only. The separate
HTTP/1.1 cache-write teardown path remains under investigation.

[JosiahWI]

The code looks good! Thank you for this fix!

[JosiahWI]

I'm adding this to the v10.2.x project in anticipation that it should be backported.

[masaori335]

[approve ci]

[JosiahWI]

The AuTest subcookie spuriously failed, and the HostDBProcessor regression test failed (unsure if this is spurious).

[saimayithri]

Thanks for the review, assignment, and for running CI !!

[JosiahWI]

Approving, but would be good to also get a review from @wuxcer.