fix(sessions): clear stale contextTokens on model switch (#38044)

Merged via squash.

Prepared head SHA: bac2df4b7f920ce271f0a15f1db9ed99b35300f3
Co-authored-by: yuweuii <82372187+yuweuii@users.noreply.github.com>
Co-authored-by: jalehman <550978+jalehman@users.noreply.github.com>
Reviewed-by: @jalehman

src/infra/git-commit.test.ts

@@ -43,6 +43,9 @@ describe("git commit resolution", () => {
 
   afterEach(() => {
     process.chdir(originalCwd);
+    vi.restoreAllMocks();
+    vi.doUnmock("node:fs");
+    vi.doUnmock("node:module");
     vi.resetModules();
   });