aft_callgraph returns 0 callers for cross-file Python call sites #1126

Reproduction (all files directly in the project root of a fresh directory, engine configured on it):

`mod.py`:

def toplevel_target():
    return 1


def local_target():
    return 2


def test_target():
    return 3


def same_file_caller():
    return toplevel_target()


`caller_toplevel.py`:

from mod import toplevel_target


def run_toplevel():
    return toplevel_target()


`caller_local.py`:

def run_local():
    from mod import local_target
    return local_target()


`tests/test_probe.py`:

class TestProbe:
    def test_local(self):
        from mod import test_target
        return test_target()


Requests and observed results:

- `{"command":"callers","file":"<root>/mod.py","symbol":"toplevel_target"}` -> `0 callers`
- `{"command":"callers","file":"<root>/mod.py","symbol":"local_target"}` -> `0 callers`
- `{"command":"callers","file":"<root>/mod.py","symbol":"test_target","includeTests":true}` -> `0 callers`
- `{"command":"call_tree","file":"<root>/caller_toplevel.py","symbol":"run_toplevel","depth":1}` ->
  `+ 1 unresolved external call: toplevel_target`
- `{"command":"call_tree","file":"<root>/caller_local.py","symbol":"run_local","depth":1}` ->
  `+ 1 unresolved external call: local_target`

After adding `same_file_caller` to `mod.py` (calls `toplevel_target` within the same file), `callers toplevel_target` returns exactly that one same-file caller — same-file Python edges resolve, cross-file Python edges do not. A TypeScript control (`ts_mod.ts` exporting `tsTarget`, `ts_caller.ts` importing `./ts_mod` and calling it) resolves the cross-file edge normally.

Expected: `callers` finds the cross-file call sites (documented as "Find all call sites of a symbol"), or — following the honest-zero-callers behavior introduced with #1015 — the reply discloses that unresolved call sites exist in the analyzed code instead of presenting a bare 0 as complete.

Observed: bare `0 callers` with no disclosure on the shipped release, while the call sites exist in plain `from`-import form (no dynamic import, no importlib/exec).