Fix #53: funnel response-shape checks through one expectKey() helper

[corgab]

Summary

Five call sites in AbstractQuantumDriver repeated the same check on decoded Python output: key present, value of the right type, else throw QuantumExecutionException::malformedResponse() with a hand-written message. Only the script, the key and the type test differed. This PR extracts the helper the issue proposes.

Plan

1. Add a private expectKey(array $response, string $script, string $key, Closure $isValid, string $expected, string $subject = 'the response'): mixed that performs the presence-and-shape check, throws the shared exception with one wording, and returns the validated value.
2. Collapse the results, per-item counts, circuit counts, task_arn, status and bits checks onto it, each passing its own predicate (is_array(...), is_string(...), non-empty string, valid TaskStatus).
3. Leave the checks that are not shape checks inline: the batch result count, a batch item that is not an object (now reported with its index), and the entropy bit length.

Changes

• src/Drivers/AbstractQuantumDriver.php: expectKey() added next to payload(); six checks rewritten as one call each. The message is uniform: expected <subject> to have a "<key>" key holding <expected>., with <subject> defaulting to the response and set to result #<n> for batch items. pollTask() hands the validated status to the snapshot instead of letting it re-parse the key. The @var annotations on counts say array<mixed>, which is what is_array() proves; CircuitResult owns the value shape as before.
• src/Tasks/TaskSnapshot.php: fromResponse() accepts an optional pre-validated TaskStatus so the guard and the snapshot agree on the value.
• tests/Unit/Drivers/AbstractQuantumDriverTest.php: assertions updated to the unified wording; new tests pin the full message for a wrong-type key, prove an absent, null and wrong-type key are all reported the same way, and name the offending batch item. tests/Unit/Tasks/TaskSnapshotTest.php: the pre-validated status is carried as given.

Tests

• vendor/bin/pint --test passes.
• vendor/bin/pest --compact: 797 tests, 1 skipped (791 before: +6).

Closes #53