Add JSON parse conformance tests for non-numeric string in numeric fields.

Adds test coverage for invalid empty strings (e.g. ""), non-numeric strings (e.g. "abc"), and partially-numeric strings (e.g. "12abc"), as well as valid exponential numeric strings ("1e5)

We will target enforcing non-conformant cases that should have failed to parse but didn't in upb in v30.x (our ~annual breaking release in some languages). Conformance failures to accept input we previously failed on can be landed at any time.

PiperOrigin-RevId: 694269337