_get_field_default_gen does not return the correct generator when freezegun is in use

The function `_get_field_default_gen` checks if a field's type is a datetime using `t is datetime`, which means if freezegun is being used in a test it generates the wrong default generator function.

Suggest this changes to `t instanceof datetime` to support cases like this.

Locate `_get_field_default_gen` in the betterproto generator and inspect how it identifies datetime field types. Reproduce the behavior with freezegun, then verify that the generated default handling is correct for both ordinary and freezegun-wrapped datetime types.

Written by the indexing model from the issue text.