feat: add labels to chats (#23594)

## Summary

Adds a general-purpose `map[string]string` label system to chats, stored
as jsonb with a GIN index for efficient containment queries.

This is a standalone foundational feature that will be used by the
upcoming Automations feature for session identity (matching webhook
events to existing chats), replacing the need for bespoke session-key
tables.

## Changes

### Database
- **Migration 000451**: Adds `labels jsonb NOT NULL DEFAULT '{}'` column
to `chats` table with a GIN index (`idx_chats_labels`)
- **`InsertChat`**: Accepts labels on creation via `COALESCE(@labels,
'{}')`
- **`UpdateChatByID`**: Supports partial update —
`COALESCE(sqlc.narg('labels'), labels)` preserves existing labels when
NULL is passed
- **`GetChats`**: New `has_labels` filter using PostgreSQL `@>`
containment operator
- **`GetAuthorizedChats`**: Synced with generated `GetChats` (new column
scan + query param)

### API
- **Create chat** (`POST /chats`): Accepts optional `labels` field,
validated before creation
- **Update chat** (`PATCH /chats/{chat}`): Supports `labels` field for
atomic label replacement
- **List chats** (`GET /chats`): Supports `?label=key:value` query
parameters (multiple are AND-ed)

### SDK
- `Chat`, `CreateChatRequest`, `UpdateChatRequest`, `ListChatsOptions`
all gain `Labels` fields
- `UpdateChatRequest.Labels` is a pointer (`*map[string]string`) so
`nil` means "don't change" vs empty map means "clear all"

### Validation (`coderd/httpapi/labels.go`)
- Max 50 labels per chat
- Key: 1–64 chars, must match `[a-zA-Z0-9][a-zA-Z0-9._/-]*` (supports
namespaced keys like `github.repo`, `automation/pr-number`)
- Value: 1–256 chars
- 13 test cases covering all edge cases

### Chat runtime
- `chatd.CreateOptions` gains `Labels` field, threaded through to
`InsertChat`
- Existing `UpdateChatByID` callers (e.g., quickgen title updates) are
unaffected — NULL labels preserve existing values via COALESCE

coderd/x/chatd/chatd.go

@@ -397,6 +397,7 @@ type CreateOptions struct {
 	SystemPrompt       string
 	InitialUserContent []codersdk.ChatMessagePart
 	MCPServerIDs       []uuid.UUID
+	Labels             database.StringMap
 }
 
 // SendMessageBusyBehavior controls what happens when a chat is already active.
@@ -475,6 +476,9 @@ func (p *Server) CreateChat(ctx context.Context, opts CreateOptions) (database.C
 	if opts.MCPServerIDs == nil {
 		opts.MCPServerIDs = []uuid.UUID{}
 	}
+	if opts.Labels == nil {
+		opts.Labels = database.StringMap{}
+	}
 
 	var chat database.Chat
 	txErr := p.db.InTx(func(tx database.Store) error {
@@ -482,6 +486,11 @@ func (p *Server) CreateChat(ctx context.Context, opts CreateOptions) (database.C
 			return limitErr
 		}
 
+		labelsJSON, err := json.Marshal(opts.Labels)
+		if err != nil {
+			return xerrors.Errorf("marshal labels: %w", err)
+		}
+
 		insertedChat, err := tx.InsertChat(ctx, database.InsertChatParams{
 			OwnerID:           opts.OwnerID,
 			WorkspaceID:       opts.WorkspaceID,
@@ -491,6 +500,10 @@ func (p *Server) CreateChat(ctx context.Context, opts CreateOptions) (database.C
 			Title:             opts.Title,
 			Mode:              opts.ChatMode,
 			MCPServerIDs:      opts.MCPServerIDs,
+			Labels: pqtype.NullRawMessage{
+				RawMessage: labelsJSON,
+				Valid:      true,
+			},
 		})
 		if err != nil {
 			return xerrors.Errorf("insert chat: %w", err)