feature: support negative expiresIn when signing tokens

[agubler]

Supports negative expiresIn option when signing tokens

Resolves #555

[Copilot]

Pull Request Overview

This PR updates the token signing logic to support negative expiresIn values, which allows generating tokens with an already passed expiration time.

• Added tests to verify behavior with negative expiresIn values in both the token verification and signing flows
• Modified validation logic in the signer to allow negative expiresIn values

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/verifier.spec.js	Added a test case to verify rejection of tokens with negative exp
test/signer.spec.js	Added a test for signing tokens with negative expiresIn and removed validation tests that expected negative values to throw errors
src/signer.js	Updated the validation logic to permit negative numbers for expiresIn

Comments suppressed due to low confidence (1)

src/signer.js:237

• Now that negative expiresIn values are supported, consider updating the error message to remove the 'positive' requirement to prevent potential confusion.

if (typeof expiresIn !== 'number' || expiresIn < 0) {

[simoneb]

LGTM, can you do a quick check if this is explicitly supported in the

[agubler]

@simoneb From a quick check there is not anything in the spec to say that negative or exp in the past are not allowed

[simoneb]

@agubler I didn't actually want to post that comment, after verifying that we're good with negative exps :)