fix: align client failure and saturation handling

[smiggleworth]

Summary

• keep Stream READ on its coded error envelope while decoding LAST and every other Stream operation as plain errors
• configure detached callback queue capacity independently from request admission and callback concurrency
• terminate KV, Notice, Queue, Lease, Schedule, and Stream callback registrations with typed AsyncHandlerOverflowError instead of silently dropping notifications
• expose subscription completion and propagate overflow through managed Queue, Schedule, and Stream iterators while retaining RPC protocol backpressure
• export Schedule ERR_BACKEND_ERROR (7010) and classify it as retryable for replay-safe operations

This is the Go parity companion to cntryl/fitz-ts#22.

Validation

• go build ./...
• go test ./... -timeout 60s
• golangci-lint run
• go test -count=1 -tags=integration ./test -timeout 10m
• four-leg TCP/WebSocket and anonymous/valid-JWT conformance matrix

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