Fix/non presigned unsigned payload

[dacut]

Supersedes #16

This adds a unit test to validate the changes here and fixes some Git weirdness with a rebase on main.

Thanks to @FrancisRussell for this contribution!

[Copilot]

Pull request overview

This PR corrects SigV4 canonicalization for unsigned payload handling on non-presigned requests by checking the x-amz-content-sha256 header (rather than query parameters), and adds coverage around presigned URL behavior. It also removes an unused constant and ensures CI runs on pull requests.

Changes:

• Fix unsigned-payload detection for non-presigned requests to read x-amz-content-sha256 from headers.
• Add presigned URL validation tests.
• Remove the unused X-Amz-Content-Sha256 query-parameter constant and run CI on pull_request events.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/canonical.rs	Switches unsigned payload detection to header-based logic; adds a presigned URL canonicalization test.
src/signature.rs	Adds an integration-style test validating a presigned URL request where the body is ignored.
src/constants.rs	Removes the unused query-parameter constant for payload SHA.
.github/workflows/rust.yml	Triggers Rust CI on pull requests in addition to pushes.

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