Fix parse_f64_approx returning NaN for 0e400

[Sebastian Turner (auraseb)]

parse_f64_approx scaled a zero mantissa by 10^exponent. For exponents above about 308 that is 0 * inf, so 0e400 parsed as NaN instead of 0.0, and the JSON parser then rejected it as invalid. A zero mantissa now skips the scaling, which also keeps the sign of -0e400.

The exponent saturates at i32::MAX, but the table index then used exponent + 3, which panicked with an overflow in debug builds for inputs like 1e2147483645. The index now uses wrapping_add, so such inputs fall through to powi and return infinity, as in release builds.

Tests: added zero_with_large_exponent and huge_exponent to float.rs and a 0e400 case to the JSON number test. All three fail on main and pass with this change. cargo test --workspace, cargo fmt --check and cargo clippy --all-targets -- --deny warnings pass.

Fixes #990

[Sebastian Turner (auraseb)]

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