feat: add organization scoping to chats (#23827)

Fixes https://github.com/coder/internal/issues/1436

* Adds organization_id to chats with backfill (workspace org → user org membership → default org)
* No support yet for ACLs (follow-up issue)
- Cross-org workspace binding rejected (both in `CreateChatRequest` and in `create_workspace` tool
- Adds `OrganizationAutocomplete` to `AgentCreateForm`
- Docs updated with `organization_id` in chats-api.md

> 🤖 Written by a Coder Agent. Reviewed by many humans and many agents.

---------

Co-authored-by: Mathias Fredriksson <mafredri@gmail.com>

coderd/httpmw/chatparam_test.go

@@ -35,7 +35,7 @@ func TestChatParam(t *testing.T) {
 		return r, user
 	}
 
-	insertChat := func(t *testing.T, db database.Store, ownerID uuid.UUID) database.Chat {
+	insertChat := func(t *testing.T, db database.Store, ownerID, organizationID uuid.UUID) database.Chat {
 		t.Helper()
 
 		_, err := db.InsertChatProvider(context.Background(), database.InsertChatProviderParams{
@@ -63,6 +63,7 @@ func TestChatParam(t *testing.T) {
 		require.NoError(t, err)
 
 		chat, err := db.InsertChat(context.Background(), database.InsertChatParams{
+			OrganizationID:    organizationID,
 			Status:            database.ChatStatusWaiting,
 			OwnerID:           ownerID,
 			WorkspaceID:       uuid.NullUUID{},
@@ -147,7 +148,8 @@ func TestChatParam(t *testing.T) {
 		})
 
 		r, user := setupAuthentication(db)
-		chat := insertChat(t, db, user.ID)
+		org := dbgen.Organization(t, db, database.Organization{})
+		chat := insertChat(t, db, user.ID, org.ID)
 
 		chi.RouteContext(r.Context()).URLParams.Add("chat", chat.ID.String())
 		rw := httptest.NewRecorder()