TaskScheduler admission scans every pending entry for the owner and burst limits

[HarryCordewener]

Raised by CodeRabbit on #1334 (#1334 (comment)).

TaskScheduler.Admit decides the owner quota by counting _pendingEntries.Values under _admissionLock (e => e.Owner == owner, already on main before #1334). #1334 adds the per-connection typed-input burst check with the same pattern. ConcurrentDictionary.Values copies, so each admission is O(pending) in time and allocation while holding the lock; filling the default 10,000-slot global queue is quadratic and serialises concurrent admissions.

Fix

Keep two counters under _admissionLock:

• charged entries per owner
• outstanding typed lines per connection incarnation (keyed the way SameIncarnation compares: handle + Metadata instance)

Increment on admit, decrement where RemoveEntry releases a reservation (and in Halt/wipe paths that drop entries). Use the counts for both limit checks; do not enumerate _pendingEntries.

Acceptance

• No _pendingEntries.Values enumeration on the admission path.
• Existing QueueAdmissionTests / QueueQuotaTests pass unchanged.
• A test proves the counts return to zero after entries complete, are halted, and are cancelled.