Implement 'fragment' style

In https://github.com/microsoft/typespec/issues/7688, I asked the Typespec team:

"A question on https://github.com/microsoft/typespec/tree/main/packages/http-specs/specs/routes :
I don't see any tests for 'fragment' style ('#' delimiter?). If that style is not deprecated, could they get added? Not sure how to implement this, specifically what is expected when explode=true. A test would clarify that. Currently, we have to throw if we see 'fragment' because we aren't sure if the code we'd emit is correct, and we'd prefer not to add such code that is not covered by tests."

So, when/if Spector team adds test for 'fragment', add support for it.

_From https://github.com/Azure/typespec-rust/issues/471#issuecomment-2989160884_
See also https://github.com/Azure/typespec-rust/pull/394#discussion_r2153363420