Added pan() and gstin() generators to en_IN provider

[RedZapdos123]

What does this change:

Adds pan() and gstin() generators to the en_IN SSN provider.

• pan() now generates values matching ^[A-Z]{5}[0-9]{4}[A-Z]$.
• gstin() now generates values matching ^[0-9]{2}[A-Z]{5}[0-9]{4}[A-Z][0-9A-Z]Z[0-9A-Z]$.
• GSTIN checksum is generated using a base-36 checksum routine over the first 14 characters.
• Adds tests for PAN format, GSTIN format, GSTIN checksum validation, and deterministic output under seeding.

What was wrong:

en_IN had aadhaar_id(), but did not provide pan() or gstin(), which are common IDs needed in India focused invoicing/compliance workflows.

How this solves it:

• Extends faker.providers.ssn.en_IN.Provider with pan() and gstin() methods.
• Adds calculate_gstin_checksum() helper in en_IN provider for checksum correctness.
• Extends TestEnIn in tests/providers/test_ssn.py with repeated generation checks (100 samples), checksum checks, and seeded reproducibility checks.
• Runs make lint (which updates stubs) so faker/proxy.pyi contains the new public methods.

Solves #2356

AI Assistance Disclosure:

• No AI tools were used in preparing this PR.
• If AI tools were used, I have disclosed which ones, and fully reviewed and verified their output.

AI tool used: GitHub Copilot (GPT-5.3-Codex), used to draft and apply code/test changes and PR text. All generated code and tests were manually reviewed and validated locally.

Checklist:

• I have read the documentation about CONTRIBUTING
• I have read the documentation about Coding style
• I have run make lint

The screenshot of the SSN provider test suite for validation (run inside WSL):

[RedZapdos123]

Data Source:

1. PAN: https://en.wikipedia.org/wiki/Permanent_account_number
2. GSTIN: https://en.wikipedia.org/wiki/Goods_and_Services_Tax_(India)