toolchain: Respect currently focused file when querying toolchains (#28875)

Closes #21743


https://github.com/user-attachments/assets/0230f233-58a4-494c-90af-28ce82f9fc1d


Release Notes:

- Virtual environment picker now looks up virtual environment based on
parent directory of active file; this enables having multiple active
virtual environments in a single worktree.

crates/project/src/project.rs

@@ -3094,6 +3094,9 @@ impl Project {
             .map(|lister| lister.term())
     }
 
+    pub fn toolchain_store(&self) -> Option<Entity<ToolchainStore>> {
+        self.toolchain_store.clone()
+    }
     pub fn activate_toolchain(
         &self,
         path: ProjectPath,