Reject non-finite values (nan, inf) in FloatField - #9998

Description

FloatField accepts non-finite floats — nan, inf, -inf, and overflow-to-inf strings such as "1e400" — returning float('nan') / float('inf') instead of raising a ValidationError. It is the only DRF numeric field without a finiteness guard: IntegerField and DecimalField already reject these, as does Django's own forms.FloatField.

serializers.FloatField().run_validation('nan')    # -> nan   (before); ValidationError (after)
serializers.FloatField().run_validation('inf')    # -> inf   (before); ValidationError (after)
serializers.FloatField().run_validation('1e400')  # -> inf   (before); ValidationError (after)

The value arrives as a JSON/form string (e.g. "nan"), so it slips past the JSON parser's parse_constant guard — float("nan") accepts it.

Why it matters

REST framework deliberately renders JSON with allow_nan=False to stay spec-compliant (rest_framework/utils/json.py). A non-finite value accepted by FloatField therefore raises ValueError: Out of range float values are not JSON compliant when the object is later serialized, turning a silent validation gap into a 500 error:

from rest_framework.renderers import JSONRenderer
JSONRenderer().render({'x': serializers.FloatField().run_validation('inf')})
# ValueError: Out of range float values are not JSON compliant: inf

Fix

Guard to_internal_value with math.isfinite, mirroring DecimalField (which rejects is_nan) and Django's forms.FloatField (which uses math.isfinite). The existing overflow path (huge integer inputs raising OverflowError) is unchanged; string inputs like "1e400" that float() turns into inf are now rejected as invalid.

Added nan, inf, -inf, 1e400, and native float('nan') / float('inf') cases to TestFloatField.invalid_inputs. Full tests/test_fields.py passes (334 passed, 3 pre-existing skips).