fix: surface subscription handler overflow

[smiggleworth]

Summary

• configure detached callback queue capacity independently from request admission and callback concurrency
• terminate all six notification subscription domains with typed AsyncHandlerOverflowException instead of silently dropping work
• expose subscription completion and propagate terminal overflow through public async iterators while retaining RPC protocol backpressure
• export Schedule ERR_BACKEND_ERROR (7010) and classify it as retryable for replay-safe operations

This is the .NET parity companion to cntryl/fitz-ts#22. Stream error decoding already matched the coded-READ and plain-non-READ contract.

Validation

• dotnet format Fitz.sln --verify-no-changes --no-restore --severity warn
• dotnet build Fitz.sln -c Release --no-restore
• dotnet test tests/Core/Core.Tests.csproj -c Release --no-build --filter FullyQualifiedName!~Integration
• dotnet pack Fitz.sln -c Release --no-restore --output artifacts/packages
• four-leg TCP/WebSocket and anonymous/valid-JWT conformance matrix
• domain workflow and invalid-JWT integration gates

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.