Accept URL-safe and unpadded base64 in from_dict for bytes fields

[gaoflow]

The proto3 JSON mapping accepts a bytes value encoded with either the standard or URL-safe base64 alphabet, with or without padding, and the reference protobuf implementation decodes all of these forms.

from_dict used plain base64.b64decode, which only handles the standard alphabet:

• URL-safe input (-/_) was silently corrupted — b64decode drops the non-standard characters, so e.g. from_dict({"v": "--__"}) returned b"" instead of the real bytes, with no error.
• Unpadded input was rejected with binascii.Error: Incorrect padding.

Fix

Normalize to the standard alphabet and restore the optional padding before decoding (betterproto2/src/betterproto2/__init__.py):

value = value.replace("-", "+").replace("_", "/")
value += "=" * (-len(value) % 4)
return b64decode(value)

Standard padded input is unchanged; URL-safe and unpadded inputs now decode correctly.

Test

tests/test_from_dict_base64.py parametrizes the four accepted forms (standard/URL-safe x padded/unpadded) plus a to_dict/from_dict round trip. Without the fix, the unpadded and URL-safe cases fail (corruption / Incorrect padding); with it, all pass.

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This change was prepared by an AI agent under my direction; I reviewed and verified it.