Security: Harden HTML event attribute filtering - refs GHSA-9wff-vv7j-p3cc

[christianbeeznest]

Summary

Hardens the HTML event attribute filter against malformed event-handler attributes that can bypass the existing RemoveOnAttributes regular expression.

The filter now:

• handles event attributes separated by /
• handles event attributes immediately following quoted attribute values
• handles repeated / separators
• keeps filtering limited to HTML tags
• preserves legitimate attribute values and plain text containing similar strings

Verification

Validated directly against RemoveOnAttributes::filter():

• 8 malformed event-attribute variants were removed
• 5 legitimate-content cases were preserved
• php -l passed
• git diff --check passed

Only the following file is changed:

src/Chamilo/CoreBundle/Component/HTMLPurifier/Filter/RemoveOnAttributes.php

The full Survey browser flow was not exercised as part of this patch.

Refs GHSA-9wff-vv7j-p3cc