Add tests for webhook endpoints and Refactor the webhooks API: Migrate from JavaScript to TypeScript and enhance type definitions

[denislavstanchev]

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[roomote]

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Review complete. Latest changes rename test files to include _test suffix and refactor the addProfileImage test to use extracted constants and more precise assertions. No new issues found.

• Update SmartsheetClient interface to use WebhooksApi type instead of any for the webhooks property
• Export webhook types from index.ts for consistency with other modules

Previous reviews

• 74d3886: Review #1
• 16891e7: Review #2
• 564fbbd: Review #3
• 5351cc7: Review #4

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