fix(sigv4): preserve equals in canonical uri paths

[jlon]

Summary

• preserve = in SigV4 canonical URI paths instead of percent-encoding it during canonical request construction
• keep canonical URI encoding for other reserved path characters such as $, space, and %
• add regression tests for both header-auth and presigned canonical requests whose object key ends with =

Related Context

• fixes the root cause behind rustfs/rustfs#2477
• this change is intended to unblock the follow-up RustFS PR in https://github.com/rustfs/rustfs/pulls

Verification

• cargo test -p s3s canonical_request_preserves_trailing_equals_in_uri_path -- --nocapture
• cargo test -p s3s presigned_canonical_request_preserves_trailing_equals_in_uri_path -- --nocapture
• cargo test -p s3s canonical_request_still_encodes_other_reserved_uri_path_characters -- --nocapture

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: bc4c45a6e6

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
crates/s3s/src/sig_v4/methods.rs	98.57% <100.00%> (+0.24%)	⬆️

... and 2 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[houseme]

Thank you for your contribution, we suggest you also submit a PR to https://github.com/s3s-project/s3s, thank you! @jlon