[storage-core] Make signed-URL expiry a validated cross-provider contract

[kang-heewon]

Priority: P1

Problem

Signed URL expiry is unconstrained in core and passed directly to each provider. Negative, NaN, and fractional values produce expired or provider-dependent URLs.

Evidence

SignedUrlOptions.expiresIn in packages/storage-core/src/libs/types.ts is unconstrained and is forwarded by the in-memory, R2, Cloudflare Images, and Cloudinary providers.

Desired outcome

Every storage adapter shares one explicit signed-URL expiry contract.

Implementation path

1. Define positive safe-integer expiry validation and a documented upper bound in storage-core.
2. Use it at every provider boundary.
3. Add a conformance table for invalid inputs and expiry propagation.

Acceptance criteria

• All providers reject the same invalid expiry values.
• Valid expiry is propagated without unit ambiguity.
• Error code and message are provider-independent.

Validation

Run tests for @croco/storage-core, @croco/storage-r2, @croco/storage-cloudflare, and @croco/storage-cloudinary.

Scope boundaries

Does not change URL signing algorithms or provider credentials.