Update Shopify Function templates to API version 2026-07

[davejcameron]

Background

Shopify Function templates using stable API versions need to move from 2026-04 to 2026-07.

Solution

• Update all 30 stable Function manifests across JavaScript, Rust, and raw Wasm templates to 2026-07.
• Regenerate their GraphQL schemas, including BuyerIdentity.shopUser, product-variant collection membership fields, and the ShopUser type.
• Align Rust fixtures with the current serialization behavior, which omits absent nullable output fields.
• Keep the 9 discounts-allocator and pickup delivery-option generator templates on unstable; Shopify does not expose those targets in the 2026-07 schema endpoint and the canonical templates still require unstable.

Validation

• JavaScript type generation and all JavaScript integration suites
• cargo fmt --check
• cargo clippy -- -D warnings
• cargo test
• cargo build --release --target wasm32-unknown-unknown
• All Rust integration suites
• JavaScript, Rust, and raw-Wasm template expansion
• Manifest/schema parity audit and git diff --check

Checklist

• I have 🎩'd these changes
• I have squashed my commits into chunks of work with meaningful commit messages