Fix managed SNI MARS receive teardown race

[cheenamalhotra]

Description

Prevent unobserved receive-continuation exceptions during managed SNI MARS teardown.

• Capture TCP and named-pipe streams once and report disposal through existing SNI errors.
• Preserve the error packet until all MARS sessions are notified; release incomplete payloads.
• Invoke error callbacks only for pending async reads and balance callback counts.
• Account for valid late callbacks and buffered reads after connection closure in debug assertions.
• Keep existing lock scope and public APIs unchanged.

Issues

Fixes #4679

Testing

Deterministic TCP/named-pipe coverage includes disposed streams, idle and pending sessions, cleared tasks, repeated errors, and packet reuse. Fixed the fake peer's unread SYN writes and write/read ordering.

31 focused unit tests and eight live SQL Server 2025 teardown cases pass on .NET 8, 9, and 10 Debug, plus .NET 9 Release. SQL cases are reported separately instead of running an opaque 200-iteration loop. Windows CI is pending for the latest correction.

Guidelines

• Tests added or updated
• Public API changes documented (none)
• Original customer harness rerun (equivalent SQL-backed regression exercised)
• Ensure no breaking changes introduced

[Copilot]

🔵 Needs a closer look

The concurrency-sensitive packet ownership changes require human review, particularly because the live SQL regression remains unexecuted.

Pull request overview

Fixes #4679 by safely handling managed SNI disposal while MARS re-arms receives.

Changes:

• Captures TCP/named-pipe streams and reports disposal as SNI errors.
• Preserves packet ownership during MARS error propagation.
• Adds transport-level and SQL-backed regression coverage.

File summaries

File	Description
SniTcpHandle.netcore.cs	Handles disposed TCP streams safely.
SniNpHandle.netcore.cs	Handles disposed named-pipe streams safely.
SniMarsConnection.netcore.cs	Corrects packet lifetime during receive failures.
SniReceiveTests.cs	Adds deterministic receive and packet-reuse tests.
MarsReceiveTeardownTest.cs	Adds SQL-backed MARS teardown coverage.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0
• Review effort level: Balanced

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[Copilot]

🔵 Needs a closer look

The concurrency-sensitive teardown and packet-ownership changes require final human review and completion of pending Windows CI.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.59%. Comparing base (ce9f01b) to head (f3d1232).
⚠️ Report is 15 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (ce9f01b) and HEAD (f3d1232). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (ce9f01b)	HEAD (f3d1232)
CI-SqlClient	1	0

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4680      +/-   ##
==========================================
- Coverage   71.88%   64.59%   -7.30%     
==========================================
  Files         290      284       -6     
  Lines       45056    68050   +22994     
==========================================
+ Hits        32389    43954   +11565     
- Misses      12667    24096   +11429     

Flag	Coverage Δ
CI-SqlClient	?
PR-SqlClient-Project	64.59% <100.00%> (?)

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