test: assert memory append failures propagate as typed EXECUTION_FAILED errors (Issue #122)

[rafaio1]

Summary

Adds tests verifying that TaskRunner.run correctly propagates memory store append failures as typed EXECUTION_FAILED errors with cause details, and preserves original RuntimeError from executor without wrapping.

Tests Added

• ✅ Throwing memory store append rejection surfaces as EXECUTION_FAILED with details.cause
• ✅ Original RuntimeError from executor preserved without double-wrapping

Verification

All 2 tests pass locally via npx vitest run tests/tasks/memory-append-failure.test.ts.

Closes #122