fix: intermittent signing error

[Darasimi-Ajewole]

Fixes #265

[Copilot]

Pull request overview

This PR fixes an intermittent signing error (issue #265) by properly handling volatile headers in AWS SigV4 request signing for Lambda@Edge functions. The root cause was that headers like "via" and "x-forwarded-for" change between network hops, causing signature mismatches.

Key Changes:

• Modified the signing logic to use unsignableHeaders option instead of manually deleting headers
• Added a new test to verify that volatile headers are excluded from the signature
• Improved test setup with proper environment variable lifecycle management

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/lambdas/sign-fn-url.ts	Updated signing logic to mark volatile headers as unsignable via the unsignableHeaders option; changed let to const for headerBag variable
src/lambdas/sign-fn-url.test.ts	Added test for volatile header exclusion; improved test setup with beforeEach/afterEach for environment variable management

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