Remove the --mode/-m CLI flag from the validation attach command to align
with the specification, which defines validation mode as an inherent
property of the validation definition set at registration time via
validation add, not as a per-attachment override.
The service layer (ToolRegistryService.attach_validation) no longer
accepts mode as a parameter; instead it reads the mode from the
validation's registered definition. The ToolRegistryRepository
_to_legacy_domain now exposes the mode field so the service can access it.
All tests that passed --mode to the CLI or service have been updated or
removed. The invalid-mode validation scenarios were removed since the
mode is no longer caller-supplied. Coverage remains at 98.7%.
ISSUES CLOSED: #913
brent.edwards