Page slugs become WooCommerceWebDriver module config

[edpittol]

Summary

Moves cart, checkout, and my-account page-slug resolution out of the database and into `WooCommerceWebDriver` module config, so browser navigation to these pages never queries the system under test. Implements the page-slug half of ADR-0008.

Implementation

• `WooCommerceWebDriver` declares three new config keys with WooCommerce-convention defaults: `cartPageSlug` (`/cart`), `checkoutPageSlug` (`/checkout`), `myAccountPageSlug` (`/my-account`). Each is independently overridable in `suite.yml`.
• `WooCommerceConfig` now holds the declared slug strings instead of taking `WPDb` and resolving page IDs/`post_name` from the database.
• `WooCommerceModuleSupport::wooCommerceConfig()` builds the config from `_getConfig()` slug keys (no memoization, so a runtime `_reconfigure` takes effect immediately). A misconfigured slug (non-string value) now throws a `ModuleException` immediately rather than silently navigating to the site root. The stale `_initialize` message about reading "WooCommerce configuration" from the DB is corrected — WPDb is still required for product/cart data assertions.

Fixes

• `CartMethods::seeCartItemQuantity` — added `waitForElement` before `grabAttributeFrom` to fix a flaky failure that occurred when the cart quantity input was not yet in the DOM at the time of the attribute grab.

Tests

• `WooCommerceConfigTest` (unit) — exposes configured slugs and per-slug overrides verbatim for all three slugs.
• `WooCommerceWebDriverPageSlugTest` (unit) — module defaults to convention slugs; each slug is independently overridable.
• `PageSlugCest` (acceptance) — navigation using cart and my-account default slugs. Checkout is omitted: WooCommerce redirects `/checkout` to `/cart` when the cart is empty, making an isolated slug assertion impossible; coverage comes from `CheckoutCest` tests that navigate with items in the cart.
• Full suite green (199 tests); PHPStan (level max) and PHPCS clean.

Acceptance criteria

• `cartPageSlug`, `checkoutPageSlug`, `myAccountPageSlug` declared with defaults `/cart`, `/checkout`, `/my-account`.
• Each slug independently overridable via `suite.yml`.
• Browser navigation reads the slug from config — no DB lookup for slugs.
• A non-string slug value throws a `ModuleException` with a clear message.
• Acceptance test covers cart and my-account default slugs (checkout covered via `CheckoutCest`).
• Existing acceptance suite stays green.

Closes #48