fix: send non-finite float properties as null instead of invalid JSON

[toby-m121]

💡 Motivation and Context

Closes #1078.

Event properties containing non-finite floats (inf, -inf, nan) were serialized with json.dumps' default allow_nan=True, producing the bare tokens Infinity/NaN. Those aren't valid JSON, so strict parsers downstream reject the whole batch and every event in it is lost.

This change makes utils.clean() coerce non-finite floats to None (sent as null), matching posthog-js, where JSON.stringify does the same. I went with coercion over raising because raising would drop the caller's entire event over one bad property.

• clean() returns None for any float that isn't math.isfinite.
• The Decimal branch now runs its converted float back through clean(). Previously it returned float(item) directly, so Decimal("Infinity") would skip the new check and still serialize as Infinity.

Since clean() already recurses through dicts, lists, tuples, sets, dataclasses and pydantic models, nested values are covered too.

💚 How did you test it?

• Added test_clean_non_finite_floats (parameterized: inf, -inf, nan, Decimal("Infinity"), plus finite float/Decimal passthrough) and test_clean_non_finite_floats_nested.
• Ran the reproduction from the issue against clean() + json.dumps(..., cls=DatetimeSerializer), including a nested Decimal("Infinity"): the payload is now {"score": null, "ratio": null, "ok": 1.5, ...} and passes a strict json.loads(..., parse_constant=...) that rejects Infinity/NaN.
• Full suite: pytest --timeout=30 → 4706 passed, 16 skipped.
• ruff format --check ., ruff check ., the mypy baseline check, and python -W error -c "import posthog" all pass.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)