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>

codersdk/chat.go

@@ -0,0 +1,153 @@
+package codersdk
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"time"
+
+	"github.com/google/uuid"
+	"github.com/kylecarbs/aisdk-go"
+	"golang.org/x/xerrors"
+)
+
+// CreateChat creates a new chat.
+func (c *Client) CreateChat(ctx context.Context) (Chat, error) {
+	res, err := c.Request(ctx, http.MethodPost, "/api/v2/chats", nil)
+	if err != nil {
+		return Chat{}, xerrors.Errorf("execute request: %w", err)
+	}
+	if res.StatusCode != http.StatusCreated {
+		return Chat{}, ReadBodyAsError(res)
+	}
+	defer res.Body.Close()
+	var chat Chat
+	return chat, json.NewDecoder(res.Body).Decode(&chat)
+}
+
+type Chat struct {
+	ID        uuid.UUID `json:"id" format:"uuid"`
+	CreatedAt time.Time `json:"created_at" format:"date-time"`
+	UpdatedAt time.Time `json:"updated_at" format:"date-time"`
+	Title     string    `json:"title"`
+}
+
+// ListChats lists all chats.
+func (c *Client) ListChats(ctx context.Context) ([]Chat, error) {
+	res, err := c.Request(ctx, http.MethodGet, "/api/v2/chats", nil)
+	if err != nil {
+		return nil, xerrors.Errorf("execute request: %w", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != http.StatusOK {
+		return nil, ReadBodyAsError(res)
+	}
+
+	var chats []Chat
+	return chats, json.NewDecoder(res.Body).Decode(&chats)
+}
+
+// Chat returns a chat by ID.
+func (c *Client) Chat(ctx context.Context, id uuid.UUID) (Chat, error) {
+	res, err := c.Request(ctx, http.MethodGet, fmt.Sprintf("/api/v2/chats/%s", id), nil)
+	if err != nil {
+		return Chat{}, xerrors.Errorf("execute request: %w", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != http.StatusOK {
+		return Chat{}, ReadBodyAsError(res)
+	}
+	var chat Chat
+	return chat, json.NewDecoder(res.Body).Decode(&chat)
+}
+
+// ChatMessages returns the messages of a chat.
+func (c *Client) ChatMessages(ctx context.Context, id uuid.UUID) ([]ChatMessage, error) {
+	res, err := c.Request(ctx, http.MethodGet, fmt.Sprintf("/api/v2/chats/%s/messages", id), nil)
+	if err != nil {
+		return nil, xerrors.Errorf("execute request: %w", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != http.StatusOK {
+		return nil, ReadBodyAsError(res)
+	}
+	var messages []ChatMessage
+	return messages, json.NewDecoder(res.Body).Decode(&messages)
+}
+
+type ChatMessage = aisdk.Message
+
+type CreateChatMessageRequest struct {
+	Model    string      `json:"model"`
+	Message  ChatMessage `json:"message"`
+	Thinking bool        `json:"thinking"`
+}
+
+// CreateChatMessage creates a new chat message and streams the response.
+// If the provided message has a conflicting ID with an existing message,
+// it will be overwritten.
+func (c *Client) CreateChatMessage(ctx context.Context, id uuid.UUID, req CreateChatMessageRequest) (<-chan aisdk.DataStreamPart, error) {
+	res, err := c.Request(ctx, http.MethodPost, fmt.Sprintf("/api/v2/chats/%s/messages", id), req)
+	defer func() {
+		if res != nil && res.Body != nil {
+			_ = res.Body.Close()
+		}
+	}()
+	if err != nil {
+		return nil, xerrors.Errorf("execute request: %w", err)
+	}
+	if res.StatusCode != http.StatusOK {
+		return nil, ReadBodyAsError(res)
+	}
+	nextEvent := ServerSentEventReader(ctx, res.Body)
+
+	wc := make(chan aisdk.DataStreamPart, 256)
+	go func() {
+		defer close(wc)
+		defer res.Body.Close()
+
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			default:
+				sse, err := nextEvent()
+				if err != nil {
+					return
+				}
+				if sse.Type != ServerSentEventTypeData {
+					continue
+				}
+				var part aisdk.DataStreamPart
+				b, ok := sse.Data.([]byte)
+				if !ok {
+					return
+				}
+				err = json.Unmarshal(b, &part)
+				if err != nil {
+					return
+				}
+				select {
+				case <-ctx.Done():
+					return
+				case wc <- part:
+				}
+			}
+		}
+	}()
+
+	return wc, nil
+}
+
+func (c *Client) DeleteChat(ctx context.Context, id uuid.UUID) error {
+	res, err := c.Request(ctx, http.MethodDelete, fmt.Sprintf("/api/v2/chats/%s", id), nil)
+	if err != nil {
+		return xerrors.Errorf("execute request: %w", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != http.StatusNoContent {
+		return ReadBodyAsError(res)
+	}
+	return nil
+}