fix: prevent command injection in example URL opening

Replace platform-specific subprocess calls with webbrowser.open() and
add URL scheme validation to the elicitation example client.

The previous Windows code path used shell=True with subprocess, which
allowed command injection via crafted URLs containing shell
metacharacters (e.g., & as a command separator in cmd.exe).

Changes:
- Remove subprocess/sys imports, use webbrowser.open() for all platforms
- Add URL scheme allowlist (http/https only) in handle_url_elicitation,
  validated before prompting the user for consent
- Return ElicitResult(action='decline') for disallowed schemes instead
  of silently continuing with action='accept'
- Simplify open_browser() to a pure browser-opening helper
- Align with the safe pattern already used in the OAuth example client

Author: maxisbey

examples/snippets/clients/url_elicitation_client.py

@@ -24,8 +24,6 @@
 
 import asyncio
 import json
-import subprocess
-import sys
 import webbrowser
 from typing import Any
 from urllib.parse import urlparse
@@ -56,15 +54,19 @@ async def handle_elicitation(
         )
 
 
+ALLOWED_SCHEMES = {"http", "https"}
+
+
 async def handle_url_elicitation(
     params: types.ElicitRequestParams,
 ) -> types.ElicitResult:
     """Handle URL mode elicitation - show security warning and optionally open browser.
 
     This function demonstrates the security-conscious approach to URL elicitation:
-    1. Display the full URL and domain for user inspection
-    2. Show the server's reason for requesting this interaction
-    3. Require explicit user consent before opening any URL
+    1. Validate the URL scheme before prompting the user
+    2. Display the full URL and domain for user inspection
+    3. Show the server's reason for requesting this interaction
+    4. Require explicit user consent before opening any URL
     """
     # Extract URL parameters - these are available on URL mode requests
     url = getattr(params, "url", None)
@@ -75,6 +77,12 @@ async def handle_url_elicitation(
         print("Error: No URL provided in elicitation request")
         return types.ElicitResult(action="cancel")
 
+    # Reject dangerous URL schemes before prompting the user
+    parsed = urlparse(str(url))
+    if parsed.scheme.lower() not in ALLOWED_SCHEMES:
+        print(f"\nRejecting URL with disallowed scheme '{parsed.scheme}': {url}")
+        return types.ElicitResult(action="decline")
+
     # Extract domain for security display
     domain = extract_domain(url)
 
@@ -124,12 +132,7 @@ def extract_domain(url: str) -> str:
 def open_browser(url: str) -> None:
     """Open URL in the default browser."""
     try:
-        if sys.platform == "darwin":
-            subprocess.run(["open", url], check=False)
-        elif sys.platform == "win32":
-            subprocess.run(["start", url], shell=True, check=False)
-        else:
-            webbrowser.open(url)
+        webbrowser.open(url)
     except Exception as e:
         print(f"Failed to open browser: {e}")
         print(f"Please manually open: {url}")