[codex] Refactor Shopify Admin API for 2026-04

[0xtlt]

Summary

• Bump the crate to 0.10.0 for a breaking Shopify Admin API refactor.
• Make GraphQL Admin API 2026-04 the minimum/default API version.
• Remove public REST support and embedded schema JSON files.
• Add modern auth modes: static access token, client credentials, expiring offline token refresh, and optional async token persistence.
• Add dynamic Admin GraphQL schema download support plus an optional CLI.
• Refactor GraphQL responses and bulk operations for Shopify 2026 APIs, including JSONL download/parsing and up to 5 concurrent bulk operations.
• Move webhook management to GraphQL and remove deprecated webhook fields.
• Update README and changelog for the 0.10.0 API.

Validation

• cargo fmt -- --check
• RUSTFLAGS='-D warnings' cargo check --all-features
• cargo test --all-features
• SHOPIFY_TEST_BULK_CONCURRENCY=1 cargo test --all-features live_five_concurrent_bulk_queries_when_enabled -- --nocapture
• cargo package --allow-dirty
• RUSTDOCFLAGS='-D warnings' cargo doc --no-deps --document-private-items --all-features

Notes

• Live integration tests remain env-gated.
• .env is ignored and was not committed.
• schema_dl.graphql still requests deprecated metadata intentionally so generated schemas include complete deprecation information.