Self-review of #34375 fix: the cleanup path ran media_pattern.sub('') over
the JSON-masked copy of the text, which baked the masking spaces into the
user-visible 'cleaned' string — a serialized tool result like
{"old":"MEDIA:/x.png"} came back as {"old":" "}.
Now mask only a length-equal copy of 'cleaned' to locate the real tag spans,
then delete those spans from the unmasked 'cleaned'. Real tags are stripped;
JSON-embedded MEDIA: text reads back verbatim. Masking 'cleaned' (not the
original 'content') keeps offsets valid after the [[audio_as_voice]] /
[[as_document]] directives are removed. Adds two cleaned-text regression tests.
fb1b681b3b