subtle cache corruption when using dataclass transform leading to `Cannot determine type of "<my-dataclass-transform>" [has-type]` and a big complicated codebase (Pomodouroboros)

Bug Report

This commit https://github.com/glyph/Pomodouroboros/commit/0d72cbb5079c87e3727a507c7752d1b053665abb prevents a mypy error, while making no semantic changes.

To Reproduce

Unfortunately I cannot construct a minimal reproducer.

1. Check out that commit, replace the call to observify(SessionManager) with @observable() above the declaration of SessionManager.
2. mypy src/pomodouroboros/model/sessions.py -> no error
3. mypy src/ ->

   src/pomodouroboros/model/sessions.py:150: error: Cannot determine type of "observable"  [has-type]
   Found 1 error in 1 file (checked 58 source files)
   

4. notice the error.

Expected Behavior

I would expect no error here, or at the very least, the same error with or without running mypy on the rest of the codebase.

Actual Behavior

See above.

Your Environment

• Mypy version used: mypy 1.19.1 (compiled: yes)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): see project
• Python version used: 3.13.12

Start with the Pomodouroboros commit and src/pomodouroboros/model/sessions.py, then run mypy src/pomodouroboros/model/sessions.py and mypy src/ as described. Compare the results with and without the @observable() change, using the reported mypy and Python versions. Done means the cache-related error is reproducible and resolved consistently for both invocation scopes.