Add non-mutating theme account alias support

[GeorgiZhelev]

What changed

• Added non-mutating account-session selection in cli-kit auth via an explicit sessionId option.
• Exposed a public findSessionIdByAlias(alias) helper for resolving account aliases without changing the current global session.
• Added shared --alias / SHOPIFY_FLAG_AUTH_ALIAS support to Shopify theme commands.
• Wired theme commands through the selected account session, including theme duplicate, theme metafields pull, silent metafields pull from theme dev, and dev-server session refreshes.
• Added a patch changeset for @shopify/cli-kit and @shopify/theme.

Why

Theme commands could only use the globally selected CLI account session. This allows commands like:

shopify theme pull --store example.myshopify.com --alias work

without mutating the user's current global CLI session.

Validation

• pnpm vitest run packages/cli-kit/src/private/node/session.test.ts packages/cli-kit/src/public/node/session.test.ts packages/theme/src/cli/utilities/theme-command.test.ts packages/theme/src/cli/utilities/theme-environment/dev-server-session.test.ts packages/theme/src/cli/services/metafields-pull.test.ts passed: 5 files, 98 tests.
• pnpm nx run theme:lint passed.
• pnpm nx run cli-kit:lint passed.
• git diff --check passed.

Notes:

• pnpm nx run cli-kit:type-check, pnpm nx run theme:type-check, and pnpm refresh-manifests currently stop in the existing cli-kit GraphiQL build/type resolution errors for h3, react-dom/server, @shopify/polaris, and @shopify/polaris-icons before reaching this change's surface.
• A sandboxed run of the metafields tests failed only because the tests write local-storage state under ~/Library/Preferences; rerunning with filesystem access passed.