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/httpapi/chatlabels.go

@@ -0,0 +1,78 @@
+package httpapi
+
+import (
+	"fmt"
+	"regexp"
+
+	"github.com/coder/coder/v2/codersdk"
+)
+
+const (
+	// maxLabelsPerChat is the maximum number of labels allowed on a
+	// single chat.
+	maxLabelsPerChat = 50
+	// maxLabelKeyLength is the maximum length of a label key in bytes.
+	maxLabelKeyLength = 64
+	// maxLabelValueLength is the maximum length of a label value in
+	// bytes.
+	maxLabelValueLength = 256
+)
+
+// labelKeyRegex validates that a label key starts with an alphanumeric
+// character and is followed by alphanumeric characters, dots, hyphens,
+// underscores, or forward slashes.
+var labelKeyRegex = regexp.MustCompile(`^[a-zA-Z0-9][a-zA-Z0-9._/-]*$`)
+
+// ValidateChatLabels checks that the provided labels map conforms to the
+// labeling constraints for chats. It returns a list of validation
+// errors, one per violated constraint.
+func ValidateChatLabels(labels map[string]string) []codersdk.ValidationError {
+	var errs []codersdk.ValidationError
+
+	if len(labels) > maxLabelsPerChat {
+		errs = append(errs, codersdk.ValidationError{
+			Field:  "labels",
+			Detail: fmt.Sprintf("too many labels (%d); maximum is %d", len(labels), maxLabelsPerChat),
+		})
+	}
+
+	for k, v := range labels {
+		if k == "" {
+			errs = append(errs, codersdk.ValidationError{
+				Field:  "labels",
+				Detail: "label key must not be empty",
+			})
+			continue
+		}
+
+		if len(k) > maxLabelKeyLength {
+			errs = append(errs, codersdk.ValidationError{
+				Field:  "labels",
+				Detail: fmt.Sprintf("label key %q exceeds maximum length of %d bytes", k, maxLabelKeyLength),
+			})
+		}
+
+		if !labelKeyRegex.MatchString(k) {
+			errs = append(errs, codersdk.ValidationError{
+				Field:  "labels",
+				Detail: fmt.Sprintf("label key %q contains invalid characters; must match %s", k, labelKeyRegex.String()),
+			})
+		}
+
+		if v == "" {
+			errs = append(errs, codersdk.ValidationError{
+				Field:  "labels",
+				Detail: fmt.Sprintf("label value for key %q must not be empty", k),
+			})
+		}
+
+		if len(v) > maxLabelValueLength {
+			errs = append(errs, codersdk.ValidationError{
+				Field:  "labels",
+				Detail: fmt.Sprintf("label value for key %q exceeds maximum length of %d bytes", k, maxLabelValueLength),
+			})
+		}
+	}
+
+	return errs
+}