`timestamp.test.ts` defines shared helper functions inline instead of importing from `helpers.ts`

Description:

In timestamp.test.ts, common helper functions plain, paragraph, bold, and strike are defined inline instead of being imported from the shared helpers.ts file used by all other test files in the same directory.

Expected Behavior

timestamp.test.ts should import plain, paragraph, bold, and strike from helpers.ts, consistent with every other test file in the package.

Current Behavior

// these are defined inline in timestamp.test.ts
const plain = (value: string) => ({ type: 'PLAIN_TEXT' as const, value });
const paragraph = (value: Array<Record<string, unknown>>) => ({ type: 'PARAGRAPH' as const, value });
const bold = (value: Array<Record<string, unknown>>) => ({ type: 'BOLD' as const, value });
const strike = (value: Array<Record<string, unknown>>) => ({ type: 'STRIKE' as const, value });

Steps to Reproduce

See packages/message-parser/tests/timestamp.test.ts

Additional Context

• Every other test file in the package imports these helpers from helpers.ts
• If the shape of a node changes in helpers.ts, the inline definitions in timestamp.test.ts could silently drift out of sync
• The fix is purely a code cleanup with no behavior change

Open packages/message-parser/tests/timestamp.test.ts and compare its helper usage with helpers.ts and the other test files in the same directory. Replace the inline plain, paragraph, bold, and strike definitions with imports from helpers.ts, then verify the timestamp tests still pass with no behavior changes.