[Structured Output] xgrammar: drop propertyNames from unsupported JSON features

[Raydius]

Summary

• Drop propertyNames from the keyword set that has_xgrammar_unsupported_json_features flags as unsupported, so JSON schemas using propertyNames are routed through xgrammar instead of being rejected/fallback-routed.
• Use a properly-formed single-element tuple (\"patternProperties\",) so the any(key in obj for key in …) check iterates over keywords rather than the characters of the string \"patternProperties\".

Why

A prior local edit on v12-dflash left the check as for key in (\"patternProperties\") — a parenthesized string, not a tuple. The membership test would iterate \"p\", \"a\", \"t\", … and return True whenever any of those characters appeared as an object key, masking the intended behavior. This PR removes propertyNames (the apparent goal of the original edit) and fixes the tuple syntax.

Test plan

• Confirm xgrammar build still imports/loads (pytest tests/v1/structured_output -k xgrammar or equivalent narrow run)
• Spot-check a JSON schema containing propertyNames passes through has_xgrammar_unsupported_json_features as supported
• Spot-check a JSON schema containing patternProperties still flags as unsupported

Notes

AI assistance was used drafting this change; the human submitter (Ray Dollete) reviewed the one-line edit and is accountable for it.

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