feat: support {{variable}} substitution in HTTP tool endpoint URL

Apply the same render_template() logic used for node prompts and preset
parameters to the endpoint URL, so users can use {{initial_context.*}}
and {{gathered_context.*}} in the URL path. The URL is rendered at
execution time just before the HTTP request.

- Backend: render URL via render_template() in execute_http_tool
- Frontend: allow {{variable}} in URL path but reject in domain
- Frontend: add hint label below Endpoint URL field
- Tests: verify URL rendering with initial_context, gathered_context,
  and static (unchanged) URLs

ui/src/app/tools/[toolUuid]/components/HttpApiToolConfig.tsx

@@ -141,6 +141,9 @@ export function HttpApiToolConfig({
 
                         <div className="grid gap-2">
                             <Label>Endpoint URL</Label>
+                            <Label className="text-xs text-muted-foreground">
+                                Supports {`{{variable}}`} syntax using initial and gathered context variables in the URL path
+                            </Label>
                             <UrlInput
                                 value={url}
                                 onChange={onUrlChange}

ui/src/components/http/url-input.tsx

@@ -19,6 +19,12 @@ export interface UrlValidationResult {
     error?: string;
 }
 
+const DOMAIN_REGEX = /^https?:\/\/([^\/]+)/;
+
+function domainContainsTemplateVar(domain: string): boolean {
+    return domain.includes("{{") || domain.includes("}}");
+}
+
 export function validateUrl(url: string): UrlValidationResult {
     const trimmedUrl = url.trim();
 
@@ -26,6 +32,18 @@ export function validateUrl(url: string): UrlValidationResult {
         return { valid: false, error: "URL is required" };
     }
 
+    // If the URL contains template variables, validate domain separately
+    if (trimmedUrl.includes("{{")) {
+        const domainMatch = trimmedUrl.match(DOMAIN_REGEX);
+        if (!domainMatch || domainContainsTemplateVar(domainMatch[1])) {
+            return {
+                valid: false,
+                error: "Invalid URL format. Template variables are only allowed in the URL path.",
+            };
+        }
+        return { valid: true };
+    }
+
     if (!URL_REGEX.test(trimmedUrl)) {
         return {
             valid: false,