refactor(shopify): hardcode Admin API version (OAuth-only config)

[guitavano]

What

The Shopify connection form is already OAuth-only — StateSchema was reduced to z.object({}) in #526, so there's no fillable domain/token/version field anymore (token comes from the OAuth Authorization header).

This PR removes the last piece of runtime-variable config: the Admin API version is now hardcoded to DEFAULT_API_VERSION (2026-07) instead of being resolved from state.apiVersion or the SHOPIFY_API_VERSION env var.

Changes

• server/types/env.ts — drop apiVersion from ShopifyCredentials; update comment.
• server/lib/client.ts — remove the state.apiVersion || SHOPIFY_API_VERSION || … resolution and the apiVersion field on MeshRequestContext.state / resolved creds; buildGraphqlUrl uses DEFAULT_API_VERSION directly.
• server/lib/client.test.ts — drop the apiVersion cases/assertions.

Notes

• Kept the local-dev env fallbacks (SHOPIFY_STORE_DOMAIN / SHOPIFY_ACCESS_TOKEN) and the legacy raw-token path — these aren't part of the connection schema and don't appear in the UI.
• All 58 tests pass.

🤖 Generated with Claude Code

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Summary by cubic

Hardcodes the Shopify Admin API version to DEFAULT_API_VERSION (2026-07) for the OAuth-only connection. Previously the version could be set via connection state or the SHOPIFY_API_VERSION env var; overrides are no longer supported to reduce config surface and prevent version drift.

• Remove any use of apiVersion in ShopifyCredentials, MeshRequestContext.state, and downstream consumers; the GraphQL URL builder always uses DEFAULT_API_VERSION.
• Stop setting SHOPIFY_API_VERSION; it has no effect.
• No changes to the UI or OAuth flow; local dev fallbacks (SHOPIFY_STORE_DOMAIN, SHOPIFY_ACCESS_TOKEN) remain.

^{Written for commit 33ea311. Summary will update on new commits.}