fix(mcp): drop orphaned path_field_renames from EndpointTool test helper

The merge with main adopted main's mcp path-substitution refactor (#10162),
which removed the `path_field_renames` field from `EndpointTool` and its
consumer (`substitute_path_params` no longer takes per-field path renames).
main's `runner.rs` `ep` test helper still constructed the struct with
`path_field_renames: None`, so the workspace test build (cargo test --all,
which compiles windmill-mcp's own #[cfg(test)] module under the `server`
feature) failed with E0560. A plain `cargo check` does not compile that test
module, so it only surfaced in CI's cargo_test.

Remove the orphaned field to match the struct.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/windmill-mcp/src/server/runner.rs

@@ -854,7 +854,6 @@ mod tests {
             path_params_schema: None,
             query_params_schema: None,
             body_schema: None,
-            path_field_renames: None,
             query_field_renames: None,
             body_field_renames: None,
         }