Add explicit type annotation to CURRENT_BUILD_KEY

[Lang-Qiu]

Problem Background

In TypeScript, explicit type annotations improve code clarity and ensure robust type checking. The exported constant CURRENT_BUILD_KEY lacked an explicit type, which could lead to less predictable type inference and potentially affect maintainability.

Changes

Added the explicit type annotation string | null to CURRENT_BUILD_KEY based on its initialization expression (GENERATED_CURRENT_BUILD_KEY || null), enhancing readability and type safety without altering functionality.

Verification

• Run the TypeScript compiler (e.g., tsc) to confirm no type errors are introduced.
• Execute project tests to ensure no behavioral changes occur.

[vercel]

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[github-actions]

No findings.

Reviewed src/lib/current-build.ts and nearby build-version usage/generation. The annotation is consistent with current behavior and does not introduce an obvious correctness, security, or performance risk.