fix: reject non-digit characters in valid_cc_number()

[gr8man]

Description
Explain what you have changed, and why.
valid_cc_number() used is_numeric() to reject non-numeric values before the Luhn check. is_numeric() accepts strings that ctype_digit() rejects — e.g. decimal points (5351367.37861108), scientific notation, and sign characters — so such input:

• bypassed digit-only validation and could pass the Luhn check (parity shifted by the skipped non-digit character), and
• reached $sumTable[...][$number[$i]] with a non-digit index, triggering an Undefined array key warning/ErrorException.

This PR switches to ctype_digit() (already used elsewhere in system/, e.g. FormatRules::valid_numeric).

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[carson-codeigniter4]

Hi there, @gr8man! 👋

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