Complete automation tasks and add bonus test coverage

[BrianJohnsonJr]

Completes all four required automation tasks plus bonus coverage. Full suite runs 9 scenarios / 38 steps.

• Login page title: corrected the expected title to "Swag Labs" to fix the failing scenario
• Login error message: added a step and page method validating the locked-out user's error banner
• Purchase flow: implemented the full checkout (cart, checkout, customer info, continue, finish, order confirmation) via new purchase.steps.ts and purchase.page.ts
• Product price sort: parameterized the price low-to-high and high-to-low sort using a Scenario Outline and Examples table
• Bonus: parameterized login errors validating "Username is required" and "Password is required" for empty credentials
• Bonus: successful login happy-path verifying a valid login reaches the inventory page with all 6 products
• Bonus: checkout required-field validation confirming the form blocks continuing without customer info
• Documented the implemented tests in the README