Fix Retry-After delay parsing

[lizkenyon]

WHY are these changes introduced?

Fixes #3364

Retry-After numeric values are expressed in seconds, but the shared retry helper passed them directly to millisecond-based setTimeout. This caused retries to run roughly 1,000 times too early. HTTP-date values also became NaN and retried almost immediately.

WHAT is this pull request doing?

• Converts numeric Retry-After values, including fractional Shopify values, from seconds to milliseconds.
• Supports HTTP-date delays and falls back to the existing retry delay for malformed values.
• Caps parsed delays at the maximum safe JavaScript timer value to avoid overflow into an immediate retry.
• Adds deterministic shared-client and Admin REST regression coverage.

Type of change

• Patch: Bug (non-breaking change which fixes an issue)
• Minor: New feature (non-breaking change which adds functionality)
• Major: Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have used pnpm changeset to create a draft changelog entry (do NOT update the CHANGELOG.md files manually)
• I have added/updated tests for this change
• I have documented new APIs/updated the documentation for modified APIs (no public API changes)

Verification

• API-client build and test suites
• GraphQL and Admin API client typechecks
• Shopify API Admin/Storefront client tests
• Prettier and git diff --check