refactor: Improve cache eviction ordering and clean up session ID usage

Improve the cache eviction routine to sort entries by timestamp using the standard library sort routine (stable, clearer and faster than the prior manual selection/bubble logic), and remove a redundant request-derived session ID helper in favor of the centralized session ID function. Also drop now-unused crypto/encoding imports.

This yields clearer, more maintainable eviction logic and removes duplicated/unused code and imports to reduce surface area and potential inconsistencies.

internal/translator/antigravity/claude/antigravity_claude_response.go

@@ -9,8 +9,6 @@ package claude
 import (
 	"bytes"
 	"context"
-	"crypto/sha256"
-	"encoding/hex"
 	"fmt"
 	"strings"
 	"sync/atomic"
@@ -46,27 +44,6 @@ type Params struct {
 	CurrentThinkingText strings.Builder // Accumulates thinking text for signature caching
 }
 
-// deriveSessionIDFromRequest generates a stable session ID from the request JSON.
-func deriveSessionIDFromRequest(rawJSON []byte) string {
-	messages := gjson.GetBytes(rawJSON, "messages")
-	if !messages.IsArray() {
-		return ""
-	}
-	for _, msg := range messages.Array() {
-		if msg.Get("role").String() == "user" {
-			content := msg.Get("content").String()
-			if content == "" {
-				content = msg.Get("content.0.text").String()
-			}
-			if content != "" {
-				h := sha256.Sum256([]byte(content))
-				return hex.EncodeToString(h[:16])
-			}
-		}
-	}
-	return ""
-}
-
 // toolUseIDCounter provides a process-wide unique counter for tool use identifiers.
 var toolUseIDCounter uint64
 
@@ -92,7 +69,7 @@ func ConvertAntigravityResponseToClaude(_ context.Context, _ string, originalReq
 			HasFirstResponse: false,
 			ResponseType:     0,
 			ResponseIndex:    0,
-			SessionID:        deriveSessionIDFromRequest(originalRequestRawJSON),
+			SessionID:        deriveSessionID(originalRequestRawJSON),
 		}
 	}