Fix query string parsing regression introduced in 2.18.2 #3106

[C0deBeez]

Fix query string parsing regression introduced in 2.18.2

This PR fixes a regression introduced in Dash 2.18.2 where query string values containing unencoded & characters were being truncated at the first &.

Problem: In 2.18.2, ?param1=something&bla&param2=something2 would result in param1='something' instead of the expected param1='something&bla' from 2.18.1.

Root Cause: The automatic URL decoding feature from #2991 caused unquote() to decode %26 to &, which parse_qs() then misinterpreted as parameter delimiters.

Solution: Remove the automatic unquote() call to restore 2.18.1 behavior while maintaining support for properly encoded URLs.

Fixes #3106

Contributor Checklist

• I have added entry in the CHANGELOG.md

Changes Made

# Before (2.18.2) - BROKEN
def _parse_query_string(search):
    search = unquote(search)   # REMOVED
    if search and len(search) > 0 and search[0] == "?":
        search = search[1:]
    else:
        parsed_qs = {}
    for (k, v) in parse_qs(search).items():
        v = v[0] if len(v) == 1 else v
        parsed_qs[k] = v
    return parsed_qs

# After (This Fix) - WORKING
def _parse_query_string(search):
    if not search or not search.startswith("?"):
        return {}

    query_string = search[1:]
    
    parsed_qs = parse_qs(query_string, keep_blank_values=True)
    
    return {
        k: v[0] if len(v) == 1 else v
        for k, v in parsed_qs.items()
    }

[gvwilson]

@T4rk1n please review

[T4rk1n]

💃 Looks good, thank you.