multi_agent/v1: add optional team_uid to Request.Metadata

[warp-agent-staging]

Summary

Adds an optional team_uid field to Request.Metadata in the multi-agent (MAA) API so Warp clients can tell the server which team an agent-mode request should be attributed to, instead of the server always guessing (teams[0]) for multi-team users.

Part of REMOTE-2723.

Changes

• apis/multi_agent/v1/request.proto: add string team_uid = 7; to Request.Metadata, documented as optional and falling back to the server's existing team resolution when unset.
• Regenerated Go bindings (apis/multi_agent/v1/gen/go/request.pb.go) via ./script/generate -a multi_agent -v v1.

Rollout ordering

This is the first of three coordinated PRs (proto -> server -> client):

1. This PR - adds the field. Non-breaking; old and new clients/servers are unaffected until the field is populated/read.
2. warp-server#14562 (depends on this): resolves team_uid when present, verifying team membership; falls back to teams[0] when absent.
3. warp#15031 (depends on Rust and Go bindings for MAA. #2): populates team_uid from the active window's team.

The server must ship and tolerate the field before any client sends it, and old clients (that never send team_uid) must keep working unchanged after the server ships.

Sensitive Fields

• I have ensured that all string fields that contain sensitive information are marked as sensitive. team_uid is an opaque team identifier, not sensitive user content.

Verification

• go build ./... in apis/multi_agent succeeds.
• Verified the only generated diff (besides request.pb.go) is the new field; other files are unchanged.

Co-Authored-By: Warp Agent agent@warp.dev