Fix/ja jp postcode leading zero

[hunhunyosshy]

What is the reason for this PR?

In Japan, some regions (notably Hokkaido) have postal codes starting with zero (e.g., 001-XXXX). The previous implementation used numberBetween(), which resulted in integers. When these were concatenated, leading zeros were lost (e.g., 001 became 1). This PR ensures leading zeros are preserved by using numerify().

• A new feature
• Fixed an issue (resolve leading zero issue in ja_JP postcode)

Author's checklist

• Follow the Contribution Guide
• New features and changes are documented

Summary of changes

• Changed postcode1() and postcode2() in Faker\Provider\ja_JP\Address to use numerify('###') and numerify('####') instead of numberBetween().
• Updated PHPDoc @example tags to reflect string return values.
• Added test/Faker/Provider/ja_JP/AddressTest.php to ensure the postcode format is correctly generated.

Review checklist

• All checks have passed
• Changes are added to the CHANGELOG.md
• Changes are approved by maintainer

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed after 1 week if no further activity occurs. Thank you for your contributions.

[hunhunyosshy]

Could you review and merge it if no problem?

[hunhunyosshy]

@localheinz

Could you review and merge it if no problem?