feat: add database tables and API routes for agentic chat feature (#17570)

Backend portion of experimental `AgenticChat` feature:
- Adds database tables for chats and chat messages
- Adds functionality to stream messages from LLM providers using
`kylecarbs/aisdk-go`
- Adds API routes with relevant functionality (list, create, update
chats, insert chat message)
- Adds experiment `codersdk.AgenticChat`

---------

Co-authored-by: Kyle Carberry <kyle@carberry.com>

coderd/database/dbauthz/dbauthz_test.go

@@ -5307,3 +5307,77 @@ func (s *MethodTestSuite) TestResourcesProvisionerdserver() {
 		}).Asserts(rbac.ResourceWorkspaceAgentDevcontainers, policy.ActionCreate)
 	}))
 }
+
+func (s *MethodTestSuite) TestChat() {
+	createChat := func(t *testing.T, db database.Store) (database.User, database.Chat, database.ChatMessage) {
+		t.Helper()
+
+		usr := dbgen.User(t, db, database.User{})
+		chat := dbgen.Chat(s.T(), db, database.Chat{
+			OwnerID: usr.ID,
+		})
+		msg := dbgen.ChatMessage(s.T(), db, database.ChatMessage{
+			ChatID: chat.ID,
+		})
+
+		return usr, chat, msg
+	}
+
+	s.Run("DeleteChat", s.Subtest(func(db database.Store, check *expects) {
+		_, c, _ := createChat(s.T(), db)
+		check.Args(c.ID).Asserts(c, policy.ActionDelete)
+	}))
+
+	s.Run("GetChatByID", s.Subtest(func(db database.Store, check *expects) {
+		_, c, _ := createChat(s.T(), db)
+		check.Args(c.ID).Asserts(c, policy.ActionRead).Returns(c)
+	}))
+
+	s.Run("GetChatMessagesByChatID", s.Subtest(func(db database.Store, check *expects) {
+		_, c, m := createChat(s.T(), db)
+		check.Args(c.ID).Asserts(c, policy.ActionRead).Returns([]database.ChatMessage{m})
+	}))
+
+	s.Run("GetChatsByOwnerID", s.Subtest(func(db database.Store, check *expects) {
+		u1, u1c1, _ := createChat(s.T(), db)
+		u1c2 := dbgen.Chat(s.T(), db, database.Chat{
+			OwnerID:   u1.ID,
+			CreatedAt: u1c1.CreatedAt.Add(time.Hour),
+		})
+		_, _, _ = createChat(s.T(), db) // other user's chat
+		check.Args(u1.ID).Asserts(u1c2, policy.ActionRead, u1c1, policy.ActionRead).Returns([]database.Chat{u1c2, u1c1})
+	}))
+
+	s.Run("InsertChat", s.Subtest(func(db database.Store, check *expects) {
+		usr := dbgen.User(s.T(), db, database.User{})
+		check.Args(database.InsertChatParams{
+			OwnerID:   usr.ID,
+			Title:     "test chat",
+			CreatedAt: dbtime.Now(),
+			UpdatedAt: dbtime.Now(),
+		}).Asserts(rbac.ResourceChat.WithOwner(usr.ID.String()), policy.ActionCreate)
+	}))
+
+	s.Run("InsertChatMessages", s.Subtest(func(db database.Store, check *expects) {
+		usr := dbgen.User(s.T(), db, database.User{})
+		chat := dbgen.Chat(s.T(), db, database.Chat{
+			OwnerID: usr.ID,
+		})
+		check.Args(database.InsertChatMessagesParams{
+			ChatID:    chat.ID,
+			CreatedAt: dbtime.Now(),
+			Model:     "test-model",
+			Provider:  "test-provider",
+			Content:   []byte(`[]`),
+		}).Asserts(chat, policy.ActionUpdate)
+	}))
+
+	s.Run("UpdateChatByID", s.Subtest(func(db database.Store, check *expects) {
+		_, c, _ := createChat(s.T(), db)
+		check.Args(database.UpdateChatByIDParams{
+			ID:        c.ID,
+			Title:     "new title",
+			UpdatedAt: dbtime.Now(),
+		}).Asserts(c, policy.ActionUpdate)
+	}))
+}