fix(fmt): stop share-URI query values from being decoded twice or dropped (#10027)

`Utils.ParseQueryString` already unescapes every value, and
`BaseFmt.GetQueryDecoded` unescaped it again. A value that still held a
valid percent sequence after the first pass decayed on the second: an
obfuscation password of `ob%41fs` is exported as `ob%2541fs` and imported
back as `obAfs`. Only well-formed sequences are affected, which is why the
damage is silent - `100%` and `66%ff` survive untouched.

The same function also split each pair on every `=`, and skipped the pair
unless exactly two halves came out. RFC 3986 lists `=` among the
sub-delimiters a query value may carry, so only the first one separates the
key from the value, and `HttpUtility.ParseQueryString` reads a query string
the same way. Splitting on all of them discarded a syntactically valid
pair: `?ech=AAj+DQAEAAAAAA==` was lost entirely, and so was a `plugin`
value in the non-canonical SIP002 spelling, since those are `;` separated
`key=value` lists.

v2rayN percent-encodes both on export, so its own links were never
affected; what changes is that the parser now follows the grammar instead
of discarding a pair it cannot split in two.

Splitting on the first `=` only, and reading the value the parser already
decoded, fixes both. `ParseQueryString` keeps decoding because
`ConfigHandler` reads its result directly.

`GetQueryDecoded` and `GetQueryValue` are now equivalent; they are left
separate to keep this change small, and can be collapsed if you prefer.

v2rayN/ServiceLib/Handler/Fmt/BaseFmt.cs

@@ -342,8 +342,13 @@ public class BaseFmt
         return query[key] ?? defaultValue;
     }
 
+    /// <summary>
+    /// Values are already unescaped by <see cref="Utils.ParseQueryString" />, so this must not
+    /// unescape them a second time: a value that still holds a valid percent sequence after the
+    /// first pass - an obfuscation password of "ob%41fs", say - would decay into "obAfs".
+    /// </summary>
     protected static string GetQueryDecoded(NameValueCollection query, string key, string defaultValue = "")
     {
-        return Utils.UrlDecode(GetQueryValue(query, key, defaultValue));
+        return GetQueryValue(query, key, defaultValue);
     }
 }