Fix greedy regex in escape_html_characters that eats text between comments

[Chessing234]

Bug

escape_html_characters in xmodule/util/misc.py is used to strip HTML
noise before ElasticSearch indexing. Its comment and CDATA strippers
misbehave in two ways:

1. Greedy: r\"<!--.*-->\" matches from the first <!-- to the
   last --> in the input. Indexable text between two separate
   comments is deleted along with the comments:

   \"<!-- a --> keep this text <!-- b -->\"
       → \"\"   (everything between the first <!-- and the last --> is gone)
   

2. Single-line only: . does not match newlines, so any multi-line
   comment / CDATA is not stripped at all:

   \"<!--\\n  notes\\n-->\" → unchanged
   

   and those spans end up in the search index.

Root cause

Both patterns use greedy .* and neither is compiled with
re.DOTALL. The helper was written assuming single-line, single-comment
input.

Why the fix is correct

• Switching to non-greedy .*? makes each re.sub match exactly one
  comment (or one CDATA span) and stop, so text between separate
  comments survives.
• Passing flags=re.DOTALL lets . also span newlines, so multi-line
  comments and CDATA blocks are stripped too.
• No other behaviour changes: the enclosing whitespace-normalisation
  re.sub is untouched, and the function's public signature is the
  same.

Change

xmodule/util/misc.py: make the two outer re.sub calls non-greedy
(.* → .*?) and add flags=re.DOTALL so multi-line comments and
CDATA are actually stripped.

[openedx-webhooks]

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[mphilbrick211]

Hi @Chessing234! Welcome, and thank you for this contribution! In order for your CLA check to turn green, you'll need to submit a CLA form. If you are contributing as an individual, please fill out the individual CLA form here.

If you are contributing on behalf of an organization, please have your manager reach out to oscm@axim.org so you may be added to your org's existing entity agreement.

Please let me know if you have any questions. Thanks!