Respect IBufferWriter leases after commit

[DeagleGross]

BufferWriter<T> retained the remainder of a span after committing bytes to the underlying IBufferWriter<byte>. Subsequent writes could therefore use an expired buffer lease and call Advance again without first calling GetSpan or GetMemory.

Clear the cached span after committing and reacquire a buffer before the next non-empty write.

Fixes #68148

[Copilot]

Copilot review overview

🟢 Approval recommended

The change directly enforces the IBufferWriter<T> leasing contract and is covered by targeted tests that validate the corrected post-commit behavior.

Review tier: Lite
Findings: None

What changed in this PR

This PR fixes a contract violation in BufferWriter<T> by ensuring it does not retain and reuse an IBufferWriter<byte>-leased span after committing (i.e., after calling the underlying writer’s Advance). This aligns BufferWriter<T> behavior with the IBufferWriter<T> contract and prevents writing into expired leases across multiple commits.

Changes:

• Clear the cached _span after Commit() publishes buffered bytes to the underlying writer, forcing reacquisition before subsequent writes.
• Ensure Write(ReadOnlySpan<byte>) reacquires a span on the first non-empty write after a commit.
• Add/adjust tests to validate the new “reacquire after commit” behavior and update PipeWriter-based expectations.

File	Description
src/​Shared/​ServerInfrastructure/​BufferWriter.cs	Clears cached span on commit and ensures reacquisition before non-empty writes after a commit.
src/​Shared/​test/​Shared.Tests/​ServerInfrastructure/​BufferWriterTests.cs	Adds a strict IBufferWriter<byte> test validating that post-commit writes require a new buffer acquisition.
src/​Servers/​Kestrel/​Core/​test/​BufferWriterWithPipeWriterTests.cs	Updates existing PipeWriter tests to reflect that Span is empty after Commit() and renames the test class accordingly.

---

💡 Add a code-review agent skill for context-aware, tailored reviews. Learn more in the docs.