Serialize negative zero floats instead of dropping them

[gaoflow]

Problem

A float or double field set to -0.0 is silently dropped during serialization. -0.0 compares equal to the 0.0 field default and is falsy, so the "is this the default value?" checks treat it as unset and skip it.

The reference protobuf implementation keeps -0.0: its sign bit is set, which distinguishes it from the default +0.0.

# google.protobuf (reference), double field v = -0.0
wire: 090000000000000080      # 9 bytes, field present
json: {"v": -0.0}

# betterproto2 (before this PR), same field v = -0.0
wire: (empty)                 # field dropped, sign lost
json: {}

So a betterproto2 producer loses the sign for any consumer (including another betterproto2 process), while +0.0 is correctly omitted. Decoding already worked: parsing the reference -0.0 wire yields -0.0, so only the encode side was affected.

Fix

Add a small _is_negative_zero helper and treat -0.0 as a non-default value in the three serialization paths that relied on value == default / not bool(value):

• binary __bytes__
• to_dict / to_json PROTO_JSON output
• to_dict PYTHON output

The real +0.0 default keeps being omitted everywhere.

Tests

tests/test_negative_zero.py checks that -0.0 survives a binary round trip and a dict/json round trip for both float and double, and that +0.0 is still omitted. The fixed binary output now matches the google reference byte for byte (090000000000000080).

Notes

I found this with a differential test of betterproto2 against the google protobuf package. AI assistance was used under my direction; I verified the behavior against the reference implementation and reviewed the change.

[AdrienVannson]

Thank you!

I see one line to change to match generated code (I don't think there is any reason to have something different here), other than that it looks great

[AdrienVannson]

Suggested change

	@dataclass(eq=True)
	@dataclass(eq=False, repr=False)

To match with standard generated messages

[gaoflow]

Done in 59614e5 — switched to @dataclass(eq=False, repr=False) to match the generated-message options. Tests still green.