fix(connections): URL import edge cases: raw special characters, Windows SQLite, IPv6 (#1315)

[ZhuchkaTriplesix]

Closes #1315

Problem (connection_url_parser.dart)

1. postgres://user:p@ss@host/db, ...:p#ss@... and mysql://root:pa/ss@host/db make Uri.tryParse return null: the user got only Invalid URL/URI format., for the commonest cause (a raw password copied from a cloud console) and with no word about encoding.
2. sqlite:///C:/data/app.db gave the path /C:/data/app.db, which does not exist on Windows.
3. uri.host is ::1 without brackets, so the default name read PostgreSQL: ::1:5432 and a Mongo URI built from the host was invalid.

Fix

• For a string Uri rejects, the user info (up to the last @ before the query) is percent-encoded, keeping what is already encoded; the row stores that valid string. If it still cannot be read and contains an @, the message says to percent-encode special characters (@ is %40, ...).
• A /X:/ or /X:\ path loses the URI's slash.
• The default name brackets an IPv6 host; MongoConnection.buildConnectionUri brackets it in the URI it writes.

Tests: four raw-password URLs (host, password, database, valid stored string), an encoded password stays encoded once, the encoding hint (and the plain message without an @), the Windows path and an unchanged POSIX path, the IPv6 name.