Avoid linked CTS for non-cancellable pipe reads

[karpinsn]

Summary

• Avoid creating a linked CancellationTokenSource when StreamPipeReader.ReadAsync receives a non-cancellable caller token.
• Pass the reader cancellation token directly on that common path while retaining linked cancellation when both tokens participate.
• Add coverage for token reuse, CancelPendingRead, caller cancellation, and a representative benchmark.

Performance

BenchmarkDotNet, .NET 8, 15 iterations with baseline/candidate/baseline:

• Mean: 248.2 ns -> 205.9 ns per read (17.1% faster)
• Allocated: 64 B -> 0 B per read
• Baseline drift: 0.83%

Validation

• Release test-project build: 0 warnings, 0 errors
• Focused StreamUseStrictPipeReaderTests.ReadAsync_*: 10 passed across net8.0 and .NET Framework

[AArnott]

I pushed an update to reuse a vs-threading extension method. I expected/hoped the perf win would be roughly the same, but with slightly less code and avoiding allocations in even more cases.
As it turned out, it's allegedly even better. I'm measuring a 25% perf improvement.

On my machine...

Before (main)

Method	Mean	Error	StdDev	Gen0	Allocated
ReadAsyncWithCancellationTokenNone	155.1 ns	1.85 ns	1.73 ns	0.0038	64 B

After (this PR with my extra commit)

Method	Mean	Error	StdDev	Allocated
ReadAsyncWithCancellationTokenNone	117.0 ns	0.35 ns	0.29 ns	-

[AArnott]

@karpinsn Thanks for your PR!

[karpinsn]

I pushed an update to reuse a vs-threading extension method. I expected/hoped the perf win would be roughly the same, but with slightly less code and avoiding allocations in even more cases. As it turned out, it's allegedly even better. I'm measuring a 25% perf improvement.

On my machine...

Before (main)

Method Mean Error StdDev Gen0 Allocated
ReadAsyncWithCancellationTokenNone 155.1 ns 1.85 ns 1.73 ns 0.0038 64 B

After (this PR with my extra commit)

Method Mean Error StdDev Allocated
ReadAsyncWithCancellationTokenNone 117.0 ns 0.35 ns 0.29 ns -

Nice! I wasn't super happy with the style of the code before, this makes it so much cleaner. Thanks!