fix(rtdb): percent-encode special characters in query values

[Shubham-Padkonde]

Query._querystr joined the JSON-encoded query values into the query string as-is, so values containing characters with a special meaning in a URL were not sent as written: equal_to('+15555555') reached the server as " 15555555", equal_to('C#') cut the query at the '#' (dropping orderBy), and '&' or '=' split the value into another parameter. These characters are now percent-encoded.

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Code Review

This pull request introduces percent-encoding for special characters (such as '#', '%', '&', '+', '=', and '?') in Firebase Realtime Database query string values to ensure they are correctly transmitted. It defines a set of safe characters and uses urllib.parse.quote to encode the values. Additionally, a comprehensive set of unit tests has been added to verify the encoding behavior. There are no review comments, and we have no feedback to provide.