test: cover ProfanityFilter edge cases (#940)

[lily1c]

What does this PR do?

Adds edge-case tests for the ProfanityFilter guardrail.

While looking at #940 I found that ProfanityFilter already has coverage — it
lives in SimpleValidatorSpec.scala in the same package rather than a
standalone ProfanityFilterSpec.scala. (Same reason the issue's pointer to
LengthCheckSpec.scala 404s: LengthCheck is covered in that combined file
too.) Rather than duplicate the file, this adds the cases that weren't yet
covered:

• caseSensitive mode combined with customBadWords — previously untested together
• withCustomWords(Set.empty) falls back to default behaviour
• tab and newline separators (split("\\s+") handles these, but it wasn't asserted)
• a bad word after leading whitespace, where the split produces an empty first token
• punctuation-attached tokens — see below

Known limitation documented, not fixed: validate splits on whitespace
only, so "badword." and "badword," don't match the word list. I've added a
test asserting current behaviour and labelled it a known limitation rather than
silently changing it. Happy to follow up with a fix (stripping punctuation, or a
word-boundary regex) if that's the preferred direction — it seemed like a
behaviour change that should be a maintainer call rather than folded into a
test-only PR.

Two items in #940's acceptance criteria also conflict with the implementation:
obfuscated variants ("F*CK", "f.u.c.k") aren't detectable given the
exact-token matching and two-word default list, and the error message
deliberately withholds the matched word for privacy — there's an existing test
asserting that.

Related issue

Relates to #940

How was this tested?

• sbt "core/testOnly *SimpleValidatorSpec" — passes
• sbt scalafmtAll — applied, no diff
• sbt test — full suite passes

Checklist

• I have read the Contributing Guide
• PR is small and focused — one change, one reason
• sbt scalafmtAll — code is formatted
• sbt test — tests pass on Scala 3
• New code includes tests
• No unrelated changes included (branched from main, not from another PR)
• Commit messages explain the "why"
• Updated CHANGELOG.md under [Unreleased] — N/A, tests only

[github-actions]

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PR Summary:

• Files changed: 1
• Additions: +32
• Deletions: -0
• Author: @lily1c

[lily1c]

@claude