Fixes #696

Root cause

TomlGenerator.writeNumber(double) and writeNumber(float) render the value with String.valueOf(...). For non-finite values that yields Java's textual forms NaN, Infinity, and -Infinity, which are not valid TOML floats. As a result the writer produces output it cannot read back — TomlMapper.readTree(...) on the writer's own output throws TomlStreamReadException: Unknown token.

Change

Map non-finite values to the TOML float tokens nan / inf / -inf (a small shared helper), leaving finite values formatted exactly as before. This is a writer-only change: the parser already accepts nan, inf, +inf, and -inf (Parser line ~389), so written non-finite values now round-trip with no reader change.

Tests

Added to TomlGeneratorTest:

• nonFiniteDoubles / nonFiniteFloats — assert exact output abc = nan|inf|-inf
• nonFiniteRoundTrip — writes NaN/±Infinity and reads them back through the same TomlMapper

All three fail before the change (two assertion failures plus a Unknown token error — the exact symptom in the issue) and pass after.

Verification done: ./mvnw -pl toml -am test -Dtest=TomlGeneratorTest — 23 tests pass with the fix; reverting only the TomlGenerator change reproduces 2 failures + 1 Unknown token error.