fix: --multipart silently drops := / :=@ JSON fields

[harsh-thakkar7]

--multipart silently dropped := and :=@ JSON fields, even though the same syntax is documented (and implemented) for --form:

$ echo '{"a": "b"}' | http --multipart POST example.com/x key:=true
# request body contained only file-upload style fields — `key` vanished entirely

SEPARATORS_GROUP_MULTIPART in httpie/cli/constants.py was missing SEPARATOR_DATA_RAW_JSON (:=) and SEPARATOR_DATA_EMBED_RAW_JSON_FILE (:=@). Request items are routed in requestitems.py's ProcessMultiItemArgs by that constant, so :=/:=@ fields were pushed into args.data — which the multipart encoder never reads:

# client.py
data, headers['Content-Type'] = get_multipart_data_and_content_type(
    data=args.multipart_data,   # ← args.data is ignored here
    boundary=...
)

The value vanished without an error, contradicting the changelog claim that JSON field syntax works with --form/--multipart.

Fix

Add both separators to SEPARATORS_GROUP_MULTIPART. The existing per-value conversion (convert_json_value_to_form_if_needed) already serializes primitives to strings and raises Cannot use complex JSON value types with --form/--multipart. for complex ones — the same path --form uses — so no other code changes were needed.

Tests

• Parametrized test_simple_json_arguments_with_non_json over ['--form', '--multipart'] — the --multipart cases fail before this change (key never reaches the server) and pass after.
• New test_simple_json_arguments_with_non_json_and_multipart_embed covering :=@ (complex value raises the documented ParseError; scalar value is forwarded — also fails before the change).

pytest -q tests/test_json.py tests/test_uploads.py tests/test_cli.py   # 364 passed

[harsh-thakkar7]

Closing this — I don't think this change is mature enough to land right now. Thanks for the project!