GST e-invoice: add synthetic validation fixtures

[sridharkalaibala]

Parent issue: #29

Goal

Add synthetic test fixtures for e-invoice readiness validation so future implementation PRs can prove field-level errors safely.

Suggested files to inspect

• api/tests/unit/
• api/src/services/
• Existing tax/invoice/sale fixture patterns

Acceptance criteria

• Fixtures include one B2B intrastate sale and one B2B interstate sale that should pass readiness checks.
• Fixtures include rejected cases for invalid GSTIN format, missing buyer GSTIN, missing HSN/SAC, missing place of supply, and mismatched tax totals.
• All names, addresses, GSTIN-like values, and invoice numbers are synthetic and clearly marked as test data.
• Tests are deterministic and do not require internet, credentials, or external APIs.
• Test names explain the operator problem each fixture represents.
• The PR links back to India GST e-invoice support: invoice readiness, IRP export contract, and safe connector boundary #29 and the offline field-contract issue.

Validation

• Run the focused API test command for the new fixture tests.
• Run cd api && npm test -- <new test file>.

Out of scope

• Real GST data.
• Live IRP/GSP submission.
• UI readiness panel implementation.