Implement support for OpenRouter sticky routing via a session_id variable

internal/llm/openai_compatible_client.go

@@ -12,6 +12,7 @@ import (
 	"os"
 	"strings"
 	"time"
+	"unicode/utf8"
 
 	"gitea.maximumdirect.net/eric/scriptorium/internal/defaults"
 	"gitea.maximumdirect.net/eric/scriptorium/internal/domain"
@@ -169,6 +170,12 @@ func openAIChatRequestFromGenerateRequest(req domain.GenerateRequest, defaultMod
 	wireReq := openAIChatRequest{
 		Model: model,
 	}
+	if sessionID := strings.TrimSpace(req.Prompt.SessionID); sessionID != "" {
+		if n := utf8.RuneCountInString(sessionID); n > domain.SessionIDMaxLength {
+			return openAIChatRequest{}, fmt.Errorf("session_id length %d exceeds maximum %d", n, domain.SessionIDMaxLength)
+		}
+		wireReq.SessionID = sessionID
+	}
 
 	wireReq.Messages = make([]openAIChatRequestMessage, 0, len(req.Prompt.Messages))
 	for _, msg := range req.Prompt.Messages {
@@ -200,6 +207,7 @@ func openAIChatRequestFromGenerateRequest(req domain.GenerateRequest, defaultMod
 
 type openAIChatRequest struct {
 	Model          string                     `json:"model"`
+	SessionID      string                     `json:"session_id,omitempty"`
 	Messages       []openAIChatRequestMessage `json:"messages"`
 	Temperature    *float64                   `json:"temperature,omitempty"`
 	MaxTokens      *int                       `json:"max_tokens,omitempty"`