[Tests] Weak or missing assertions allow latent bugs to pass CI

[s2x]

Context / Problem

Several tests assert too weakly to catch real defects:

tests/Integration/BasicCrudTest.php:153 — getAddressesForKey only asserts assertNotEmpty($addresses). An array of FFI\CData objects is still "not empty", so it masks the wrong-return-type bug in FutureStringArray.
tests/Integration/DirectoryTest.php:363-374 — the partition subdirectory test never writes/reads a key through the returned subspace, so the partition prefix layout is never validated.

Affected files / locations

• tests/Integration/BasicCrudTest.php:145-154
• tests/Integration/DirectoryTest.php:362-374

Proposed approach

Strengthen assertions to verify behavior, not just shape: assert assertContainsOnly('string', ...) for addresses; in partition tests, set a value via the returned subspace and assert the exact byte prefix of the stored key. Audit the rest of the suite for assertions that do not exercise the claimed behavior.

Acceptance Criteria

• Documentation updated/added (testing conventions note)
• Unit test(s) added/strengthened where applicable
• Functional test(s) added: address-type assertion and partition key-prefix round-trip
• Changelog entry added
• Post-implementation comment posted on this issue describing what was changed

Severity

Medium