PrincetonUniversity / PsyNeuLink / 28273519593

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Commit Message

Fix FitzHughNagumoIntegrator reset crash and value-shape inconsistency in a Mechanism (#3560)

## Summary

Fixes two related bugs affecting `FitzHughNagumoIntegrator` (and
multi-stateful integrators generally) when used inside a `Mechanism`.
Both surfaced from the reproducer below.

### Reproducer

```python
import psyneulink as pnl

inp = pnl.ProcessingMechanism(input_shapes=1)
lc = pnl.TransferMechanism(
    input_shapes=1,
    integrator_mode=True,
    integrator_function=pnl.FitzHughNagumoIntegrator(),
    function=pnl.Linear(),
    output_ports=[{pnl.VARIABLE: (pnl.OWNER_VALUE, 1)}],
    reset_stateful_function_when=pnl.AtTrialStart(),
)
comp = pnl.Composition()
comp.add_nodes([inp, lc])
comp.add_projection(sender=inp, receiver=lc)
comp.run(inputs={inp: [[1.0]]})   # IndexError in Python mode; worked in LLVMRun
```

## Bug 1 — `IndexError` on reset (`mechanism.py`)

On reset, the Mechanism rebuilt its value from the integrator's reset
output but kept only element `[0]`:

```python
new_input = self.integrator_function.reset(...)[0]
```

`StatefulFunction.reset()` returns one entry per stateful attribute. For
single-attribute integrators (`AdaptiveIntegrator` → `[previous_value]`)
the `[0]` correctly unwraps. For `FitzHughNagumoIntegrator` → `[v, w,
time]` it discarded `w` and `time`, leaving the value with a single
element, so an `OutputPort` selecting `(OWNER_VALUE, 1)` indexed past
the end → `IndexError`. Compiled mode was unaffected (statically-typed
value buffer; reset only rewrites the integrator state).

**Fix:** keep all stateful terms when there is more than one.

## Bug 2 — value-shape inconsistency between Python and compiled
(`integratorfunctions.py`)

Even after Bug 1, the mechanism's value shape differed between modes:
Python produced `(3, 1, 1)`, compiled `(3, 1)` (and `defaults.value` was
`(3, 1)`). FHN stacks its three stateful terms; in a Mechanism,
`variable` is the Mechanism's 2d variable `(1, 1)` while the stateful
values are 1... (continued)

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