Fix IPv6 Zone ID decoding to correctly handle RFC 6874 %25 separator

Per RFC 6874, an IPv6 Zone ID in a URI is encoded as:
  IPv6addrz = IPv6address "%25" ZoneID

So in 'http://[fe80::1%251]/', the zone ID is '1', not '251'.

Previously, _encode_host() split the host on bare '%', treating '251'
as the zone ID.  The host property also returned the raw (encoded)
value unchanged for IP addresses, so %25 was never decoded.

Fix _encode_host() to partition on '%25' (RFC 6874 separator) when
present, preserving it verbatim in raw_host / str(url), and update
the host property to decode '%25' -> '%' so callers receive the
human-readable zone identifier (e.g. 'fe80::1%1' / 'fe80::1%eth0').

Tests added for:
- Numeric zone ID: http://[fe80::1%251]/  -> host='fe80::1%1'
- String zone ID:  http://[fe80::1%25eth0]/ -> host='fe80::1%eth0'

Author: rodrigobnogueira

tests/test_url.py

@@ -373,6 +373,14 @@ def test_ipv4_zone() -> None:
     assert url.raw_host == SplitResult(*url._val).hostname
 
 
+def test_ipv6_zone_rfc6874() -> None:
+    url = URL("http://[fe80::1%251]/")
+    assert url.raw_host == "fe80::1%251"
+    assert url.host == "fe80::1%1"
+    assert url.host_subcomponent == "[fe80::1%251]"
+    assert str(url) == "http://[fe80::1%251]/"
+
+
 def test_port_for_explicit_port() -> None:
     url = URL("http://example.com:8888")
     assert 8888 == url.port

yarl/_url.py

@@ -764,6 +764,9 @@ def host(self) -> str | None:
             return None
         if raw and raw[-1].isdigit() or ":" in raw:
             # IP addresses are never IDNA encoded
+            if "%25" in raw:
+                # Decode RFC 6874 zone ID separator
+                return raw.replace("%25", "%")
             return raw
         return _idna_decode(raw)
 
@@ -1550,7 +1553,10 @@ def _encode_host(host: str, validate_host: bool) -> str:
     # If the host ends with a digit or contains a colon, its likely
     # an IP address.
     if host and (host[-1].isdigit() or ":" in host):
-        raw_ip, sep, zone = host.partition("%")
+        if "%25" in host:
+            raw_ip, sep, zone = host.partition("%25")
+        else:
+            raw_ip, sep, zone = host.partition("%")
         # If it looks like an IP, we check with _ip_compressed_version
         # and fall-through if its not an IP address. This is a performance
         # optimization to avoid parsing IP addresses as much as possible
@@ -1578,8 +1584,8 @@ def _encode_host(host: str, validate_host: bool) -> str:
             # LRU to keep the cache size small
             host = ip.compressed
             if ip.version == 6:
-                return f"[{host}%{zone}]" if sep else f"[{host}]"
-            return f"{host}%{zone}" if sep else host
+                return f"[{host}{sep}{zone}]" if sep else f"[{host}]"
+            return f"{host}{sep}{zone}" if sep else host
 
     # IDNA encoding is slow, skip it for ASCII-only strings
     if host.isascii():