fix(webtransport): harden session I/O

[ryanfowler]

Summary

• handle partial and zero-progress WebTransport stream writes without silently truncating data
• make stream and datagram cancellation preserve the initiating error and stop pending input cleanly
• report output write failures precisely and include the current QUIC limit for oversized datagrams
• add race-focused coverage for cancellation, short I/O, error precedence, and terminal flush failures

Validation

• go test -v -parallel 8 ./...
• go test -race -count=100 ./internal/wt
• staticcheck ./...
• go build .
• go mod tidy && go mod verify
• three independent review passes; final pass found no actionable issues