fix(http): retain shared buffer across attempts

[thomhurst]

Summary

• keep request-content buffering alive for the full request execution instead of per-attempt waiter lifetimes
• cancel pending buffering when the overall execution completes or caller cancellation fires
• make the hedge/attempt-timeout race deterministic in the integration test

Test plan

• deterministic regression fails before fix with 2 serializations
• dotnet build Kevlar.slnx -c Release
• dotnet run --project tests/Kevlar.IntegrationTests -c Release --no-build -- --timeout 5m (123 passed)
• dotnet run --project tests/Kevlar.NetStandard.Tests -c Release --no-build -- --timeout 5m (10 passed)
• focused hedge buffering regression repeated 10/10 successfully

Benchmark

BenchmarkDotNet ShortRun, Windows 11, .NET 10.0.11, HttpReplayBenchmarks.BufferedContent_WithRetry:

	Mean	Allocated
Before	2.247 μs	3.17 KB
After	1.959 μs	3.16 KB

No throughput or allocation regression observed (~13% lower mean in this short local run).

Closes #248

[coderabbitai]

Important

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Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 27abb4c1-d058-4436-9922-0498d92d5c24

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