Include the "Z" in the Indian GST number check character

[kevin9327]

Motivation / Background

This Pull Request has been created because the numbers generated by Faker::Company.indian_gst_number don't pass the GSTIN checksum. The 15th character is a check character computed over the 14 characters before it, but the generator only feeds it the state code, PAN and entity number (13 characters) and leaves out the fixed Z in position 14.

For example, the commonly cited real GSTIN 27AAPFU0939F1ZV validates with the standard algorithm (alternating weights 1/2, base 36, over the first 14 characters). Faker output like 98EPAJT7470W1ZM gets a different check character from that algorithm (N here), so it fails any validator that checks the checksum.

I found this while running Faker's checksum-carrying generators (IBAN, CPF/CNPJ, RUT, OIB, NIP, VIN, barcodes, ...) through independent validators. This was the only one with a wrong checksum.

Additional information

The fix adds z_char to the computed block's dependencies and includes it in the checksummed string. The new test recomputes the check character from the first 14 characters of a few generated numbers. It fails before the change and passes after. test_faker_company.rb and RuboCop pass locally.

#3327 also edits indian_gst_number (the PAN digits and the test regex) but doesn't touch the checksum, so the two changes are independent.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug, refactor something, or add a feature.
• Tests and Rubocop are passing before submitting your proposed changes.