feat: add request response batching

[eventhorizon-cli]

Summary

• Add first-class typed request/response batching with ordered response completion.
• Restrict batcher registration to explicit handler types and remove delegate registrations.
• Keep design documentation current-state-only and migrate tests and benchmarks to handler types.
• Preserve the existing pipeline when a duplicate response registration is rejected.

Breaking change

AddRequestBatcher now accepts handler types only. Move delegate logic into an IRequestBatchHandler or IRequestBatchHandler<TRequest, TResponse> implementation, then register that type with an explicit ServiceLifetime.

Validation

• dotnet format RequestBatcher.slnx --no-restore --include <changed C# files>
• dotnet build RequestBatcher.slnx --configuration Release --no-restore --tl:off -m:1 -p:UseSharedCompilation=false
• dotnet test tests/RequestBatcher.Tests/RequestBatcher.Tests.csproj --configuration Release --no-build --no-restore
• dotnet pack src/RequestBatcher/RequestBatcher.csproj --configuration Release --no-build --no-restore

[codecov]

Codecov Report

❌ Patch coverage is 95.12195% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.49%. Comparing base (bfa75c5) to head (69cbc9a).

Files with missing lines	Patch %	Lines
src/RequestBatcher/RequestBatchItem.cs	85.71%	1 Missing and 1 partial ⚠️
src/RequestBatcher/RequestBatchPartitionKey.cs	83.33%	2 Missing ⚠️
src/RequestBatcher/ResponseRequestBatcher.cs	92.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #3      +/-   ##
==========================================
+ Coverage   89.06%   90.49%   +1.43%     
==========================================
  Files          10       15       +5     
  Lines         649      747      +98     
  Branches       95      106      +11     
==========================================
+ Hits          578      676      +98     
  Misses         42       42              
  Partials       29       29              

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