* MM-68976 Preserve PluginSettings.SignaturePublicKeyFiles on config patch endpoint
The full PUT /api/v4/config endpoint silently preserves
PluginSettings.SignaturePublicKeyFiles (added in #13682), but the sparse
PUT /api/v4/config/patch endpoint had no equivalent guard, so a session
with sysconsole_write_plugins could modify the field through it.
Mirror the full update endpoint's behavior by silently preserving the
existing value in patchConfig, and add a regression test equivalent to
the one in TestUpdateConfig.
* MM-68976 Document SignaturePublicKeyFiles as non-modifiable in config API spec
Both the update and patch config endpoints preserve
PluginSettings.SignaturePublicKeyFiles; note this in the OpenAPI
descriptions alongside the existing PluginSettings.EnableUploads note.
