Implement support for OpenRouter sticky routing via a session_id variable

internal/prompt/go_renderer.go

@@ -6,7 +6,9 @@ import (
 	"errors"
 	"fmt"
 	"gitea.maximumdirect.net/eric/scriptorium/internal/domain"
+	"strings"
 	"text/template"
+	"unicode/utf8"
 )
 
 var (
@@ -50,6 +52,11 @@ func (r *goRenderer) Render(ctx context.Context, definition *domain.PromptDefini
 		},
 	}
 
+	sessionID, err := renderSessionID(definition.SessionID, funcs, vars)
+	if err != nil {
+		return nil, err
+	}
+
 	var renderedMessages []domain.RenderedMessage
 
 	for i, tmplMsg := range definition.Templates {
@@ -82,10 +89,33 @@ func (r *goRenderer) Render(ctx context.Context, definition *domain.PromptDefini
 	}
 
 	return &domain.RenderedPrompt{
-		Messages: renderedMessages,
+		SessionID: sessionID,
+		Messages:  renderedMessages,
 	}, nil
 }
 
+func renderSessionID(raw string, funcs template.FuncMap, vars map[string]string) (string, error) {
+	if strings.TrimSpace(raw) == "" {
+		return "", nil
+	}
+
+	tmpl, err := template.New("session_id").Funcs(funcs).Option("missingkey=error").Parse(raw)
+	if err != nil {
+		return "", fmt.Errorf("%w: session_id: %v", ErrInvalidTemplate, err)
+	}
+
+	var buf bytes.Buffer
+	if err := tmpl.Execute(&buf, vars); err != nil {
+		return "", fmt.Errorf("%w: session_id: %w", ErrRenderFailure, err)
+	}
+
+	sessionID := strings.TrimSpace(buf.String())
+	if n := utf8.RuneCountInString(sessionID); n > domain.SessionIDMaxLength {
+		return "", fmt.Errorf("%w: session_id length %d exceeds maximum %d", ErrRenderFailure, n, domain.SessionIDMaxLength)
+	}
+	return sessionID, nil
+}
+
 func cloneCacheControl(in *domain.CacheControl) *domain.CacheControl {
 	if in == nil {
 		return nil