Regenerated the Go code for the stream record envelope.

[moetemp]

This PR regenerates the Go code for the stream record envelope.

What changed?

• .gitmodules points proto/api at the moetemp/api fork, and the pin moves to the envelope commit in Added the stream record envelope to the public API. api#6.
• Everything else is what the pin forces: the new stream/v1 package with StreamRecord and StreamRecordKind.
• The payload visitor in proxy/interceptor.go gets a StreamRecord case for body and metadata. Nothing reaches it yet, so no visitor count moves.

Part of AI-198 (epic AI-37).

Why?

It's the Go half of the envelope api PR. Go code that reads or writes stream records through a provider needs the type. Nothing here is hand-written. It sits on main now, like the api PR, since the channel regen it used to stack on is paused per the 2026-10-05 design review.

How did you test it?

Link to a test plan if any -

• Unit Tests
• Staging
• End to End Tests

make proto ran with the same plugin versions as before, except the nexus-gen step: its input, the Nexus yaml in proto/api, doesn't change with the envelope, so its output stays as committed. The generated files and their changed lines match the regen in the earlier version of this PR. make check, go build ./... and go test -tags protolegacy ./... pass.