fix(json-enc): restore hx-vals keys dropped by empty-array FormData encoding

[alter]

Description

encodeParameters builds object from the request's FormData parameters, then walks Object.keys(object) to restore original types from hx-vals/hx-vars. A key whose value serializes to nothing in FormData (an empty array, since there's nothing to push into the form data for it) never gets an entry in object in the first place, so that restore loop never visits it and the key is silently dropped from the JSON body.

Reported against htmx core in bigskysoftware/htmx#3845, but the actual code is here: the json-enc extension's own restore step only looks at keys FormData already produced, not at the full set of keys in hx-vals.

Fix

Restore over the union of Object.keys(object) and Object.keys(vals) instead of only Object.keys(object), so a key present in hx-vals but absent from the FormData-derived object still gets added.

Testing

Added keeps hx-vals keys whose value serializes to nothing in FormData, mirroring the reporter's repro (hx-vals="js:{'firstLevel': [], 'secondLevel': {'values': []}}"). Confirmed it fails against the unmodified code (firstLevel missing from the parsed request body) and passes after the fix. Full suite: 13/13 passing, no regressions.

Closes bigskysoftware/htmx#3845

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