fix(proxy): add missing make_sticky_selector function

Tests imported make_sticky_selector but it was never implemented.
Hash-based (MD5) consistent selector — same key always returns the
same proxy, distributes across the pool.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

extract/padelnomics_extract/src/padelnomics_extract/proxy.py

@@ -57,6 +57,27 @@ def make_round_robin_cycler(proxy_urls: list[str]):
     return next_proxy
 
 
+def make_sticky_selector(proxy_urls: list[str]):
+    """Hash-based sticky proxy selector.
+
+    Returns a callable: select_proxy(key: str) -> str | None
+    The same key always maps to the same proxy (consistent hashing).
+    Returns None-returning callable if no proxies configured.
+    """
+    if not proxy_urls:
+        return lambda key: None
+
+    n = len(proxy_urls)
+
+    def select_proxy(key: str) -> str:
+        import hashlib
+
+        idx = int(hashlib.md5(key.encode(), usedforsecurity=False).hexdigest(), 16) % n
+        return proxy_urls[idx]
+
+    return select_proxy
+
+
 def make_tiered_cycler(
     primary_urls: list[str],
     fallback_urls: list[str],