feat: tag chat-originating agent logs with chat_id (#25019)

Workspace-agent logs emitted while serving chatd-driven requests were
not correlated with the originating chat, making agent logs hard to
attribute to the corresponding/originating chat.

This adds agent-side chat context middleware that parses `Coder-Chat-Id`
once, enriches agent access logs and structured handler/background logs,
and adds a chatd bridge log when chat headers are attached to an agent
connection.

Closes CODAGT-324

agent/agentchat/log.go

@@ -0,0 +1,85 @@
+package agentchat
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/google/uuid"
+
+	"cdr.dev/slog/v3"
+	"github.com/coder/coder/v2/coderd/httpmw/loggermw"
+)
+
+type chatContextKey struct{}
+
+// Context carries the chat identity associated with an agent request.
+type Context struct {
+	ID          uuid.UUID
+	AncestorIDs []uuid.UUID
+}
+
+// FromContext returns the chat identity stored on the context.
+func FromContext(ctx context.Context) (Context, bool) {
+	chatCtx, ok := ctx.Value(chatContextKey{}).(Context)
+	if !ok || chatCtx.ID == uuid.Nil {
+		return Context{}, false
+	}
+	return chatCtx, true
+}
+
+// WithContext stores chat identity on the context for downstream logs.
+func WithContext(ctx context.Context, chatID uuid.UUID, ancestorIDs []uuid.UUID) context.Context {
+	if chatID == uuid.Nil {
+		return ctx
+	}
+	ancestors := make([]uuid.UUID, len(ancestorIDs))
+	copy(ancestors, ancestorIDs)
+	return context.WithValue(ctx, chatContextKey{}, Context{
+		ID:          chatID,
+		AncestorIDs: ancestors,
+	})
+}
+
+// Fields returns structured log fields for the chat identity on ctx.
+func Fields(ctx context.Context) []slog.Field {
+	chatCtx, ok := FromContext(ctx)
+	if !ok {
+		return nil
+	}
+	return chatFields(chatCtx.ID, chatCtx.AncestorIDs)
+}
+
+// Middleware tags agent logs for requests that originate from
+// chatd. Agent log lines emitted while serving a request with Coder-Chat-Id,
+// or by background work started by such a request, should include chat_id.
+// Install after loggermw.Logger so access-log enrichment can run.
+func Middleware(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
+		chatID, ancestorIDs, ok := extractContext(r)
+		if !ok {
+			next.ServeHTTP(rw, r)
+			return
+		}
+
+		fields := chatFields(chatID, ancestorIDs)
+		if requestLogger := loggermw.RequestLoggerFromContext(r.Context()); requestLogger != nil {
+			requestLogger.WithFields(fields...)
+		}
+
+		ctx := WithContext(r.Context(), chatID, ancestorIDs)
+		next.ServeHTTP(rw, r.WithContext(ctx))
+	})
+}
+
+func chatFields(chatID uuid.UUID, ancestorIDs []uuid.UUID) []slog.Field {
+	fields := []slog.Field{slog.F("chat_id", chatID.String())}
+	if len(ancestorIDs) == 0 {
+		return fields
+	}
+
+	ancestors := make([]string, 0, len(ancestorIDs))
+	for _, id := range ancestorIDs {
+		ancestors = append(ancestors, id.String())
+	}
+	return append(fields, slog.F("ancestor_chat_ids", ancestors))
+}