Formatting: Preserve '=' characters in add_query_arg() values

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/50279

This PR improves the regex in add_query_arg() to only strip the = sign
for truly empty values (e.g., foo= → foo), while preserving valid =
characters that appear inside query values (such as Base64 padding xxxxx==).

• Updated regex from #=(&|$)# to /((?<=^|&)[^=]+=)(&|$)/ to ensure only
  empty values are matched.
• Retains backward compatibility: empty values remain normalized.
• Prevents corruption of values ending in =.

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