parsePartialJson

Parse possibly-partial JSON. Mirrors Vercel AI SDK v6 parsePartialJson:

• null input ->PartialJsonState.UndefinedInput (note: an empty/blank string is NOT undefined-input — it falls through and fails).

• raw text parses ->PartialJsonState.SuccessfulParse.

• fixJson-repaired text parses ->PartialJsonState.RepairedParse.

• otherwise ->PartialJsonState.FailedParse.

The repair runs on the ORIGINAL raw text, not on intermediate output.