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/python/README.md

@@ -86,6 +86,17 @@ dark_screenshot = snap.capture(
 )
 ```
 
+### Custom User Agent
+
+```python
+# Set a custom User-Agent string for the request
+screenshot = snap.capture(
+    "https://example.com",
+    user_agent="Mozilla/5.0 (compatible; SnapAPI/1.0)",
+    format="png",
+)
+```
+
 ### Hide Elements Before Capture
 
 ```python