test: add AC8/AC9 unit tests for ARMO auth header injection

Extract armoHeaders closure to package-level buildAuthHeaders() so it
can be tested directly. Add tests verifying X-API-Key and X-Customer-GUID
are present for ARMO endpoints (AC8) and absent for non-ARMO ones (AC9)
across all three exporter option builders (trace, log, metric).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>