feat: add userAgent parameter for custom User-Agent headers

- Add userAgent?: string to ScreenshotOptions interface
- Implement validation (max 500 chars, no newlines for security)
- Call page.setUserAgent() after page acquisition, before navigation
- Add route handler support for both POST body and GET query
- Add comprehensive test coverage (11 new tests)
- Update OpenAPI documentation with parameter specs and examples
- Update Node.js and Python SDK README examples
- All userAgent tests passing (414 → 425 total tests)

Fixes potential HTTP header injection by rejecting newlines.
Enables custom User-Agent strings for specific browser emulation needs.

sdk/node/README.md

@@ -83,6 +83,17 @@ const darkScreenshot = await snap.capture({
 });
 ```
 
+### Custom User Agent
+
+```typescript
+// Set a custom User-Agent string for the request
+const screenshot = await snap.capture({
+  url: 'https://example.com',
+  userAgent: 'Mozilla/5.0 (compatible; SnapAPI/1.0)',
+  format: 'png',
+});
+```
+
 ### Hide Elements Before Capture
 
 ```typescript