Integration tests: add coverage for path workflow and signature

[NewGraphEnvironment]

Problem

Integration tests exercise mc_send() with html = directly and sig = FALSE throughout. Missing coverage:

• No integration test sends from a .md file via path = (the primary user workflow)
• No integration test uses sig = TRUE (default signature appended)
• The mc_compose() + mc_scroll() table test exists but could verify the rendered HTML more thoroughly

Proposed Solution

Add integration tests that:

1. Send an email using mc_send(path = "file.md", ...) to exercise the full markdown → HTML → MIME pipeline
2. Send at least one email with sig = TRUE to confirm signature renders in a real Gmail message