Fix core Messages test after API refactor

[mikea]

Motivation

• The Messages helper API was refactored to use array-backed construction and pushAll/get() access, which caused the existing test to call removed methods and fail; the test was updated to match the new API while preserving the original behavioral assertions.

Description

• Updated packages/core/test/messages.test.ts to construct Messages with an initial array and append with pushAll(...) instead of the removed push(...) usage.
• Replaced the old replace(...)/slice(...) assertions with in-place mutation via messages.get().splice(...) and messages.get().slice(...) to verify the same in-place replacement behavior against the new API.
• Renamed the test description to reflect the updated approach (replacing the internal array in-place).

Testing

• Ran pnpm --filter @piccolo/core test and the test suite for @piccolo/core passed (all tests green).
• Ran the full workspace tests with pnpm -r test and they passed (all projects/tests green).
• Ran pnpm check (type/lint checks) and it completed successfully.

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Codex Task