ProgramSet MeasuredEntry.expectation() returns None when no observable is attached (unimplemented)

### Summary
`MeasuredEntry.expectation()` on a `ProgramSetQuantumTaskResult` returns `None` (and warns `"No observable was measured"`) whenever the entry has no attached observable, even though the entry contains the `measurements` needed to compute the expectation. The code path is explicitly unimplemented.

### Location
`src/braket/tasks/program_set_quantum_task_result.py`, `MeasuredEntry` (verified present on `main` as of 2026-07):
```python
self._expectation = (
expectation_from_measurements(counts, measured_qubits, self.observable, self.observable.targets)
if self.observable
else None
)

def expectation(self) -> float | None:
# TODO: Use program set payload to calculate expectation
if self._expectation is None:
warnings.warn("No observable was measured", stacklevel=1)
return self._expectation
```

### Impact
Downstream consumers that request an expectation from a program-set result receive `None`/no usable value and can silently produce wrong results, despite the measurements being present and correct. We hit this via the PennyLane–Braket plugin, where `qml.expval` returns values off by up to ~1.5 for a program-set result on a QPU (filing a companion issue on that repo, will cross-link). Because the failure is silent (only a `UserWarning`) and the measurements look fine, the corruption is easy to miss, in our case it manufactured a spurious ~0.2 R² effect that looked like a hardware-fidelity difference.

### Suggested fix
Implement the `# TODO`: when `self.observable` is absent but the requested observable is a computational-basis/diagonal observable (or is recoverable from the program-set payload), compute the expectation from `measurements` instead of returning `None`.

### Environment
Reproduced on `amazon-braket-sdk==1.113.1`; the unimplemented path is still present on `main` (1.121.0). Device: Rigetti Cepheus-1-108Q. The measurement bitstrings on the entry are correct, only the computed expectation is missing.