fix(engine): bind MCP callers to the canonical workspace [skip ci]

Canonicalize the selected plugin workspace before pool comparison, storage
and fork construction. Equivalent symlink paths retain the existing caller
and attachment guards instead of rejecting a real session.

Adapt the merged PR #7138 existing shutdown test to the extracted app-server
and runtime crate paths; production shutdown behavior is unchanged.

Verified on native23 ordinary macOS ARM64 debug build with all 4,249 source
inputs unchanged: sealed selected checks 389 passed, 0 failed; merged shutdown
completion cases 11 passed, 0 failed. Native22 real DeepSeek Flash symlink
workspace read completed successfully; production MCP postimage is identical.
Native23 P04 v4 and actual PostgreSQL pg7 joined observers passed. Actual
Stats observer passed on native22. These are local/source/provider receipts,
not Windows/package, release, or deployment qualification.

Refs CW-7764. Evidence: CW/artifacts/takeover-20261103/ and
CW/artifacts/engine-0103-execution-20261102/coherent-native23-existing-v1/.

crates/app-server/src/exec_agent.rs

@@ -1920,9 +1920,9 @@ mod tests {
 
     #[test]
     fn run_completion_exec_shutdown_consumes_real_spawned_engine() {
-        let _env = crate::test_support::lock_test_env();
+        let _env = codewhale_engine::test_support::lock_test_env();
         let home = tempfile::tempdir().unwrap();
-        let _home = crate::test_support::EnvVarGuard::set("CODEWHALE_HOME", home.path());
+        let _home = codewhale_engine::test_support::EnvVarGuard::set("CODEWHALE_HOME", home.path());
         let runtime = tokio::runtime::Builder::new_current_thread()
             .enable_all()
             .build()
@@ -1940,7 +1940,7 @@ mod tests {
                 .disable(codewhale_runtime::features::Feature::Mcp);
             let handle = codewhale_engine::core::engine::spawn_engine(
                 config,
-                &crate::config::Config::default(),
+                &codewhale_runtime::config::Config::default(),
                 None,
             )
             .await;

crates/engine/src/mcp.rs

@@ -4062,10 +4062,11 @@ impl McpPool {
         &mut self,
         plugins: Arc<codewhale_plugins::PluginRegistry>,
     ) -> Result<()> {
+        let workspace = checked_workspace_path(plugins.workspace())?;
         anyhow::ensure!(
             self.workspace
                 .as_deref()
-                .is_none_or(|workspace| workspace == plugins.workspace()),
+                .is_none_or(|current| current == workspace),
             "another MCP workspace must use its own selected pool fork"
         );
         let next = plugins.caller_selection();
@@ -4073,7 +4074,7 @@ impl McpPool {
             .plugin_registry
             .as_deref()
             .and_then(codewhale_plugins::PluginRegistry::caller_selection);
-        if next == previous && self.workspace.as_deref() == Some(plugins.workspace()) {
+        if next == previous && self.workspace.as_deref() == Some(workspace.as_path()) {
             return Ok(());
         }
         anyhow::ensure!(
@@ -4100,7 +4101,7 @@ impl McpPool {
             self.drop_all_connections("caller selection changed");
             self.config_hash = hash_mcp_config(&config);
             self.config = config;
-            self.workspace = Some(plugins.workspace().to_path_buf());
+            self.workspace = Some(workspace);
             self.plugin_registry = Some(plugins);
             self.catalog_generation.fetch_add(1, Ordering::SeqCst);
         }
@@ -4114,7 +4115,7 @@ impl McpPool {
         &self,
         plugins: Arc<codewhale_plugins::PluginRegistry>,
     ) -> Result<Self> {
-        let workspace = plugins.workspace().to_path_buf();
+        let workspace = checked_workspace_path(plugins.workspace())?;
         let config = match self.config_sources.first() {
             Some(primary) => {
                 load_config_with_workspace_and_plugins(primary, &workspace, plugins.as_ref())?