dmypy's `--use-fine-grained-cache` crashes with `KeyError` on namespace packages

This is possibly related to #10735.

To Reproduce & Traceback

Create a minimal project as follows:

project/
  mypy.ini
      [mypy]
      files = src/
  src/
    pkg/
      __init__.py
      nspkg/
        mod.py
            a: int = ""

Reproduce with these steps:

1. Navigate to project/

2. $ mypy --cache-fine-grained
   src/pkg/nspkg/mod.py:1: error: Incompatible types in assignment (expression has type "str", variable has type "int")  [assignment]
   

3. $ dmypy run -- --use-fine-grained-cache
   Daemon started
   Success: no issues found in 2 source files
   

   (I don't think this should say Success given the errors from Step 2., but that's a separate issue.)
4. Change mod.py, e.g. modify the sole line to a: int = "a"
5. Run the daemon twice:

   • $ dmypy run -- --use-fine-grained-cache
     src/pkg/nspkg/mod.py:1: error: Incompatible types in assignment (expression has type "str", variable has type "int")  [assignment]
     

   • $ dmypy run -- --use-fine-grained-cache
     Daemon crashed!
     Traceback (most recent call last):
       File ".../mypy/dmypy_server.py", line 237, in serve
         ...
       File ".../mypy/dmypy_server.py", line 768, in find_reachable_changed_modules
      worklist.append(BuildSource(graph[dep].path, graph[dep].id, followed=True))
                                  ~~~~~^^^^^
     KeyError: 'pkg.nspkg'
     

Your Environment

• Mypy version used: 1.19.1
• Python version used: Python 3.13

Reproduce the namespace-package case with dmypy and --use-fine-grained-cache, then inspect dmypy_server.py around find_reachable_changed_modules and line 768, where the traceback looks up pkg.nspkg in graph. Use the listed commands to verify the daemon no longer crashes on the second run and still reports the assignment error.