feat(plugins): drop plugin API v2

A V2 plugin passed the version gate and then failed silently while
registering its ribbon, since the ribbon wire types have moved on. The
host now loads V3 and newer, so V2 plugins are rejected up front with a
version message. The V2 compatibility fixture crate goes too.

Fixes #1085
Fixes #987

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/ocs_plugin_api/src/version_info.rs

@@ -148,7 +148,10 @@ mod tests {
             Some(env!("CARGO_PKG_VERSION"))
         );
         assert_eq!(value["api_version"].as_i64(), Some(crate::API_VERSION as i64));
-        assert_eq!(value["api_version_min_supported"].as_i64(), Some(2));
+        assert_eq!(
+            value["api_version_min_supported"].as_i64(),
+            Some(crate::API_VERSION_MIN_SUPPORTED as i64)
+        );
 
         // acadrust_version should have a patch component (e.g. "0.4.0").
         let opencadcodec = value["acadrust_version"].as_str().expect("acadrust_version string");