test: snapshot invoice PDF structure and totals

[Ebomi]

Overview

This PR adds test coverage for generateInvoicePdf in src/services/invoicePdf.ts, which previously built a customer-facing PDF Buffer from InvoicePdfData with no tests. The tests snapshot the PDF structure (header/footer), assert that line item totals are reflected in the output, and cover zero-line-item and special-character edge cases. A small hardening change to the PDF text escaping was needed to satisfy the special-character criterion.

Related Issue

Changes

🧾 Invoice PDF tests

• [ADD] src/services/invoicePdf.test.ts
  • Asserts the generated buffer starts with the %PDF- header and ends with %%EOF.
  • Asserts the invoice number and computed total appear in the PDF content.
  • Asserts each line item total matches the input (quantity × unit price).
  • Covers the empty line-item list case and asserts a valid PDF is still produced.
  • Covers parentheses and backslashes in API names and asserts the document is not corrupted (header/footer intact, escaped text present, no stray unescaped delimiters).

🛡️ PDF text escaping

• [MODIFY] src/services/invoicePdf.ts
  • Escapes (, ), and \ in text drawn into the PDF content stream so API names containing these characters cannot break the document structure.
  • No change to the public signature of generateInvoicePdf or to InvoicePdfData; existing callers are unaffected.

Verification Results

npm test -- src/services/invoicePdf.test.ts
✅ all tests passed

Acceptance Criteria	Status
Output begins with the PDF header and ends with %%EOF	✅ Asserted in invoicePdf.test.ts
Line item totals match the input	✅ Per-item totals asserted against quantity × unit price
Parentheses and backslashes in names do not corrupt the document	✅ Escaping added in invoicePdf.ts; covered by test
Empty line-item lists still generate a valid PDF	✅ Zero-line-item case asserted to produce header/footer-valid output

Security and Failure Modes

• Injection into the PDF content stream: Unescaped (, ), or \ in API names could terminate or corrupt a text object. Escaping these characters in invoicePdf.ts prevents malformed output and keeps the document parseable.
• Empty input: The zero-line-item path is exercised so a missing/empty lineItems array cannot silently produce a broken buffer.
• No weakened validation: No safeguards were removed; the only production change is additive escaping.

Compatibility

• generateInvoicePdf signature and InvoicePdfData shape are unchanged.
• Escaping only affects text rendering for names containing (, ), or \; names without these characters render identically.
• No dependency or config changes.

Closes #1301

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