fix: Checkout notices further implemented

[kidunot89]

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What does this implement/fix? Explain your changes.

Fix checkout mutation notices handling (Issue #666)

Problem: The checkout mutation wasn't exposing WooCommerce session notices, causing them to persist across requests and preventing clients from receiving payment gateway error messages.

Solution:

• Added notices field to checkout mutation output with new CartNotice and CartNoticeTypeEnum GraphQL types
• Enhanced error handling to capture WC notices in catch blocks and include them in error messages
• Ensured notices are properly cleared from session to prevent persistence
• Updated test expectations for WooCommerce error message format changes

Changes:

• includes/mutation/class-checkout.php: Enhanced mutation with notice handling
• includes/type/enum/class-cart-notice-type.php: New enum for notice types
• includes/type/object/class-cart-notice.php: New GraphQL object for notices
• includes/class-type-registry.php: Registered new types
• includes/class-wp-graphql-woocommerce.php: Added type imports
• tests/wpunit/CheckoutNoticesTest.php: Comprehensive test suite
• tests/wpunit/CartMutationsTest.php: Fixed failing test with updated error message

Testing: All 5 new checkout notice tests pass, plus fixed existing cart mutation test.

Does this close any currently open issues?

Fixes #666

Any relevant logs, error output, GraphiQL screenshots, etc?

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