fix: nan inf float

[davidhewitt]

Change Summary

Based upon the first commit from #888 (many thanks @JeanArhancet), adding a config value ser_json_inf_nan to choose whether to serialize as null or constants.

Unlike the last comment in #888 we decided not to have the serializer interact with allow_inf_nan at all. We preferred to not error on the serialization path, and just assume that if allow_inf_nan=False then the validator has already prevented an out of range value.

Probably in V3 we want to flip the default of ser_json_inf_nan to constants, as that matches closer the behaviour of json.dumps.

Related issue number

Closes #872
Closes #888

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq]

CodSpeed Performance Report

Merging #1062 will not alter performance

_{Comparing dh/ser-nan-inf-float (c62b637) with main (cb47b96)}

Summary

✅ 140 untouched benchmarks

[davidhewitt]

I understand that @samuelcolvin is happy with the design points here, so will merge this so we can release in 2.5

[davidhewitt]

(implementation can always be changed later)