validate digits in numeric string exponent

[vapebw]

In php 8.x, strings with an exponent indicator (e/E) require at least one digit after the exponent or sign to be considered numeric strings. Strings like "0e", "1e", or "1e+" must be treated as non-numeric.

Previously, Value.isNumericString in src/runtime/value.zig consumed the exponent marker without verifying that digits followed this caused false numeric comparisons such as:

• "0e" == 0 returning true
• "1e" == "1" returning true
• "0e" <=> "0" returning 0 instead of 1

While stdlib/types.zig's native is_numeric() already rejected these strings, Value.isNumericString() was missing the check

Solution

• Require at least one digit after the exponent marker (e/E and optional +/-) in Value.isNumericString
• Return false immediately if no exponent digits were consumed

Tests

• Added unit tests in src/runtime/value.zig for valid and invalid exponent cases
• Added parity test in tests/numeric_string_exponent.php matching native php 8.x behavior

[nvms]

Confirmed the bug against PHP 8.5 and ran the unit and compat suites on the branch. Thanks for the fix and the tests.