fix(sdk): support prefixed Upstash store credentials

[svllvsx]

Summary

• Accept the store-prefixed Upstash URL and token in the keeper heartbeat and position snapshot loaders, matching the sibling stores.
• Preserve unprefixed precedence, trimming, and in-memory fallback behavior, including explicitly blank credentials.
• Add isolated regression coverage for both naming schemes, independently missing values, precedence, and local/deployed fallback logging with mocked Redis requests.

Test plan

• pnpm lint && pnpm typecheck && pnpm test pass locally (1,927 tests on current main).
• pnpm format:check, pnpm typecheck:api, and pnpm coverage pass.
• All 73 heartbeat/snapshot tests pass. Six regression cases fail against the original loaders and pass with the fix.
• Vercel build and API-local build pass.

Closes #1057

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[collinsezedike]

@svllvsx thank you for the contribution. There are more open issues if you would like to keep going.

[collinsezedike]

@svllvsx thank you again for getting this over the line. If you are enjoying working on Meridian, starring the repository helps more contributors find the project and is much appreciated.