refactor: add API client

[joshsny]

First step of a refactor. We're utilising various functions to interact with the API. Let's pull this out into an API client we can test better.

Adds

• An API client
• Removes the existing posthogApi.ts functions
• Integration tests you can run locally to test the API client

[Copilot]

Pull Request Overview

This PR introduces an ApiClient class to refactor the codebase away from individual API functions toward a centralized client pattern. The refactor improves testability by consolidating API interactions into a single client with comprehensive integration tests.

• Replaces scattered API functions with a structured ApiClient class that provides organized methods for different API endpoints
• Adds comprehensive test infrastructure with Vitest configuration and integration tests covering all major API operations
• Updates the main application code to use the new client pattern while maintaining existing functionality

Reviewed Changes

Copilot reviewed 10 out of 12 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/api/client.ts	Implements the new ApiClient class with methods for organizations, projects, feature flags, insights, dashboards, queries, and users
tests/api/client.test.ts	Comprehensive integration tests covering all ApiClient functionality with proper setup and cleanup
src/index.ts	Refactors existing code to use the new ApiClient instead of individual API functions
vitest.config.ts	Test configuration for the new testing infrastructure
tests/setup.ts	Test environment setup loading test-specific environment variables
package.json	Adds testing dependencies and scripts

Files not reviewed (1)

• pnpm-lock.yaml: Language not supported

Comments suppressed due to low confidence (1)

tests/api/client.test.ts:367

• Skipped test reduces coverage for SQL insight functionality. Consider implementing the test or adding a comment explaining why it's skipped.

		it.skip("should execute SQL insight query", async () => {

[jonathanlab]

Great stuff 🚢