test(mcp): cover per-definition callers truncation

[bompus]

Summary

codegraph_callers only groups by definition when the same name has more than one definition. The truncation test on main used a single hot function, so the grouped distinct definitions / … +N more assertion failed even though the flat Showing 20 of 26 marker is correct for that fixture.

This change gives hot two definitions so that assertion matches the product path. Handler behavior is unchanged.

Test plan

• fnm exec --using codegraph npx vitest run __tests__/mcp-callers-truncation.test.ts --minWorkers 1 --maxWorkers 1 (5/5 pass)

[bompus]

Closing as superseded: #1862 landed the same per-definition truncation coverage with a stronger fixture (exact-named hot.ts + other-hot.ts, pinning that a filename is not an overload per #1809). The assertions here are identical to what is now on main, so there is nothing left to carry over. Thanks for the review bandwidth.