GetLAPSPassword: fix LAPS v2 password JSON parse

[psycep]

Follow-up from testing #37. Once LAPS v2 decryption worked end-to-end, the tool still failed to parse the recovered password with failed to parse decrypted JSON, leaving the password unusable in the output.

Root cause. The decrypted LAPS v2 blob is UTF-16LE JSON. The parse path scanned the raw UTF-16LE bytes backwards for } (0x7D) and truncated there — but } encodes as the two bytes 7D 00, so it kept the low byte and dropped the high 00. utf16ToString then couldn't pair the orphaned byte and discarded it, silently deleting the closing brace and breaking every parse.

Fix. Decode the full UTF-16LE plaintext (utf16ToString already stops at the first NUL, so NUL-terminated trailing metadata is dropped) and parse with a streaming json.Decoder, which reads exactly one object and ignores any trailing metadata that isn't NUL-delimited. Removes the byte-level brace scan entirely.

Test. Adds a regression test covering the brace-splitting case plus NUL-terminated, trailing-metadata, and password-containing-} variants.

Validation. Verified end-to-end against a live Windows LAPS v2 (encrypted / MS-GKDI) host — the tool now prints the LAPS username and password cleanly where it previously errored.

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