feat(tests): implement unit tests and DB mocking for Hub REST API

[Vaishnav88sk]

📖 Description

This PR addresses Issue #19 by introducing comprehensive unit tests for the SRE Hub REST API.

To ensure the CI tests can run quickly and securely without requiring a live PostgreSQL instance, we integrated go-sqlmock to intercept and mock all database queries at the database/sql layer.

✅ Changes Made

• Dependency Addition: Added github.com/DATA-DOG/go-sqlmock to the sre-agent/hub module.
• Database Refactor: Introduced a new NewWithDB(db *sql.DB) constructor in internal/db/store.go to safely inject mocked connections without triggering schema migrations.
• API Tests: Created internal/api/handlers_test.go utilizing net/http/httptest to test our endpoints.

🧪 Testing Scope

The new test suite fully covers the acceptance criteria defined in #19:

• TestReceiveIncident_Valid: Verifies a 201 Created response when a valid incident payload is submitted and database expectations are met.
• TestReceiveIncident_InvalidJSON: Verifies a 400 Bad Request error is returned when malformed JSON is posted.
• TestReceiveIncident_Unauthorized: Verifies a 401 Unauthorized error when the X-Claritty-Key header is missing.
• TestGetStats: Verifies that the metrics calculations return a 200 OK JSON response.

Fixes #19

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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