fix: reject callback names colliding between nested state bodies

[fgmacedo]

Fixes #653

Why

Callbacks declared inside State.Compound / State.Parallel bodies are flattened into the class namespace keyed by name only, so the same name in two sibling regions kept only the last body. With the issue's example, send("go") moved both regions and ran 1 of the 2 go bodies (log ['r2'], measured on origin/main). The same loss applied to plain callbacks such as on_enter_<state> in two bodies and to a nested body shadowing a method of the class body (log ['nested']).

What changes

• A callback name declared in two nested bodies raises InvalidDefinition at class creation, naming the callback and both state bodies. The decorator form reports the name the user wrote (go, not the mangled _go_).
• A nested callback that shadows a method of the class body raises too (today it silently replaced it).
• A nested callback whose name an event, or another attribute of the class, already uses raises with a message naming the event or the attribute. Before, this clobbered the event trigger silently, or crashed with TypeError for states.
• The same callable assigned under one name in several bodies stays accepted: plain function, bound method of one instance, functools.partial.
• Subclassing is unaffected: the check only sees callbacks pending on a state, and a subclass that overrides a nested callback in its own body keeps working. The subclass crash on _callbacks is Subclassing a StateChart with callbacks in a State.Compound body raises AttributeError #646, handled by its own PR; this check does not misfire with either del or = None there.
• Namespacing each nested body so the same name can be reused across regions stays with Read every statechart class body through one reader with an owning state #657.

Evidence

Issue reproduction, before and after:

$ uv run python before_after.py   # origin/main factory.py
log: ['r2']
$ uv run python before_after.py   # this branch
InvalidDefinition: Callback 'go' is declared in both the 'r1' and the 'r2' state bodies. Nested state bodies share the class namespace. Rename one of them.

$ timeout 120 uv run pytest tests/test_statechart_compound.py -k TestCallbackNameCollision -p no:cacheprovider --no-cov -q
16 passed, 51 deselected in 0.09s
$ timeout 120 uv run pytest -n 4 -p no:cacheprovider -q
2358 passed, 1 skipped, 44 xfailed, 9 warnings in 24.53s
$ timeout 120 uv run pytest tests/test_statechart_compound.py --cov=statemachine.factory --cov-report=term-missing --cov-branch -p no:cacheprovider -q
statemachine/factory.py  ...  (no missing line or branch inside the new code, 166-224)
$ uv run mypy --namespace-packages --explicit-package-bases statemachine/ tests/
Success: no issues found in 262 source files
$ uv run pyright statemachine/
0 errors, 0 warnings, 0 informations

The seven raising tests fail on origin/main (7 failed, 8 passed when the test class runs against a git archive origin/main export).

Three adversarial review rounds by a fresh reviewer each. Round 1 found a bound-method false positive (is comparison) and misleading "class body" wording for events and metaclass attributes, both fixed in the second commit. Round 2 found a TypeError from States.__eq__ when the clashing attribute is cls.states, fixed in the third commit. Round 3 verified the guard only.

Not verified

• A subclass that adds a new nested body reusing a callback name from a base body is not detected (the base state has no pending callbacks on the second metaclass run). Left as "a subclass overrides on purpose".
• Two distinct callables of the same type whose __eq__ returns True across instances pass the check and the last one wins. Not reachable with functions, methods or partials.
• Helper classes or Enums defined inside a nested body are classified as callbacks by pre-existing code, so two bodies defining class Kind(Enum) raise with "Callback 'Kind'"; the collision is real, only the noun is off.
• Not run against the parallel PR for Subclassing a StateChart with callbacks in a State.Compound body raises AttributeError #646 itself; the state._callbacks = None variant was simulated locally by patching the line.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (dbf8eb0) to head (aec7d91).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #674   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           53        53           
  Lines         5714      5787   +73     
  Branches       900       918   +18     
=========================================
+ Hits          5714      5787   +73     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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[sonarqubecloud]

Quality Gate passed

Issues
11 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
5.0% Duplication on New Code

See analysis details on SonarQube Cloud