build(deps): bump actions/checkout from 4 to 7

[bearyung]

What changed

• Update both actions/checkout uses in the provider-runtime workflow from v4 to v7.

Why

• Dependabot PR build(deps): bump actions/checkout from 4 to 7 #95 carried the same update but became conflicted after the adjacent setup-node and artifact-action upgrades merged.
• This replacement applies the checkout upgrade cleanly on the current combined dependency head.

Impact

• No application code changes.
• The manual provider-runtime workflow uses the current Node 24-based checkout action on GitHub-hosted runners.

Validation

• Workflow YAML parses successfully.
• git diff --check passes.
• The combined npm dependency updates already passed 1,055 Vitest tests, 96 Node script tests, Svelte check (0 errors, 0 warnings), and the production build.