add: LinkedIn username validation (social category)

Uses Twitterbot User-Agent to bypass LinkedIn's anti-bot detection
(HTTP 999). LinkedIn whitelists social media crawler bots for link
previews, serving proper responses: 200/301 for existing profiles,
404 for non-existing ones.

Author: SylvainM98

user_scanner/user_scan/social/linkedin.py

@@ -0,0 +1,19 @@
+from user_scanner.core.orchestrator import status_validate
+from user_scanner.core.result import Result
+
+
+def validate_linkedin(user: str) -> Result:
+    # LinkedIn whitelists social media crawler bots for link previews,
+    # bypassing the anti-bot 999 response that blocks regular user agents.
+    # Note: bulk scanning may trigger rate limiting (999) after ~3 rapid requests.
+    url = f"https://www.linkedin.com/in/{user}"
+    show_url = "https://linkedin.com"
+
+    headers = {
+        "User-Agent": "Twitterbot/1.0",
+    }
+
+    return status_validate(
+        url, available=404, taken=[200, 301], show_url=show_url,
+        headers=headers, follow_redirects=False
+    )