feat(schema): expose stream writer close signal

[Zhang-986]

What type of PR is this?

feat

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More detailed description

This exposes the existing receiver-close notification on StreamWriter.

Problem:
A stream producer can currently observe reader closure only when Send returns closed=true. If the producer is blocked outside Send (for example reading a stalled SSE/network stream), it cannot observe that the consumer has called StreamReader.Close(), so the producer goroutine and upstream connection may remain alive until an external context cancels.

Root cause:
The stream already has an internal closed channel closed by closeRecv, but StreamWriter does not expose it.

Change:
Add StreamWriter.Closed() <-chan struct{} so producers can select on receiver closure and cancel blocked upstream work.

Scope:
This only exposes the existing close signal. It does not change Send, Close, buffering, copy, merge, or automatic-close behavior.

Which issue(s) this PR fixes:

Fixes #1068

Test

• go test ./schema -run TestStreamWriterClosedClosesWhenReaderCloses -count=1
• go test ./schema
• go test ./... -count=1
• git diff --check