Fix DNS-over-QUIC stream lifetime under cancellation

[zs311521]

@ShreyasZare This fixes a deterministic DNS-over-QUIC stream-retention path reproduced while the peer's bidirectional-stream capacity is exhausted.

On current .NET 11 preview 6 with Microsoft MsQuic 2.5.9, cancelling OpenOutboundStreamAsync while it waits for peer capacity disposes an incompletely opened QuicStream. That requests native ABORT_SEND | ABORT_RECEIVE | IMMEDIATE (0xE); under sustained traffic, repeated timed-out queries leave native streams retained until the peer or connection catches up.

The change:

• uses StreamCapacityCallback to maintain per-connection managed admission permits, so caller cancellation happens before a native stream is opened;
• owns any admitted native open through terminal completion, preserves the original caller token and absolute query deadline, and observes late work;
• serializes connection retirement and reports cleanup failures;
• makes pooled connection leases and physical eviction explicit/single-flight so expired pooled connections are actually torn down;
• keeps hot pool hits concurrent while cold construction remains single-owner.

Deterministic A/B verification against current develop:

• stock: 202 native streams at peak with two intentionally held streams;
• patched: peak stayed at the two held streams and returned to zero;
• 1,000/1,000 caller cancellations preserved the original token;
• native start/close/release/free traces balanced, with zero caller-driven 0xE;
• 50 pool churn rounds across TCP/TLS/HTTPS/QUIC left no entry behind, and concurrent cleanup joined one result/failure;
• five interleaved 60,000-query runs showed no throughput regression (36,645 stock vs 38,290 patched median QPS in the clean-branch run);
• Release build on the exact preview-6 SDK: 0 warnings, 0 errors.

This is the concrete DoQ/native-lifetime mechanism observed in the forwarder-mode investigation related to TechnitiumSoftware/DnsServer#2030. It is deliberately not presented as an explanation for every report in that broader issue.