test: raise coverage on under-tested input/protocol packages

[aheritier]

Summary

Priority coverage gaps (TUI intentionally excluded — TUIViewPurity cop + manual QA deemed sufficient):

1. pkg/reference (0% — parses user input): parse round-trips / fuzz.
2. pkg/config/latest (6.4% direct): validate.go edge cases.
3. pkg/gateway (23.8%): catalog cache / ETag paths.
4. pkg/acp (20.9%) / pkg/a2a (23.4%): protocol adapters.

Fix

Add focused tests for the above, prioritizing pkg/reference (fuzz) and pkg/config/latest validate.

Acceptance

• Meaningful coverage increase on the listed packages; new tests deterministic.