test: add coverage for origin callback edge cases

[mahmoodhamdi]

What

Added 7 tests covering origin callback behaviors that were previously untested, despite 100% line coverage.

Why

While working with cors in my Express APIs, I was reading through the test suite and noticed several origin callback edge cases that lacked explicit test coverage. Even though line coverage was already at 100%, these behavioral scenarios weren't being validated:

• Origin parameter forwarding — verifies the callback receives the actual req.headers.origin value
• Error propagation — verifies errors from the origin callback are correctly forwarded to next(err)
• String return value — verifies that returning a specific string (not just true/false) from the origin callback sets the correct Access-Control-Allow-Origin and Vary headers
• Preflight with regexp origin — verifies OPTIONS requests work correctly when origin is a RegExp
• Preflight with array origin — verifies OPTIONS requests work correctly when origin is an array of checks
• Preflight with origin: true — verifies OPTIONS requests reflect the request origin and include default methods
• Methods as string — verifies that passing methods as a comma-separated string (not just an array) works on preflight

Testing

56 passing (42ms)

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------|---------|----------|---------|---------|-------------------
All files |     100 |      100 |     100 |     100 |
 index.js |     100 |      100 |     100 |     100 |
----------|---------|----------|---------|---------|-------------------

• All 56 tests pass (49 existing + 7 new)
• Lint passes
• 100% coverage maintained