Fix RichTextField tag-stripping feature is polynomial and does not strip unfinished tags

[WiXSL]

Summary

• replace the tag-stripping regex with a linear tag scanner in RichTextField
• keep the existing stripTags behavior while handling unfinished tags more safely
• add a regression test covering malformed input

Why

GitHub code scanning reports two alerts on RichTextField:

• #56 Incomplete multi-character sanitization
• #57 Polynomial regular expression used on uncontrolled data

Both alerts came from the regex used by removeTags().

Testing

• yarn test-unit --runTestsByPath packages/ra-ui-materialui/src/field/RichTextField.spec.tsx