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

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[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

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[houseme]

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

[houseme]

s3s-project#589

[houseme]

Closing this because main already contains the broader upstream fix from s3s-project/s3s#589 via commit 928550a (fix(s3s): allow raw URI path SigV4 fallback (#589)). Compared with this PR, #589 keeps the standard canonical URI behavior and adds a narrow raw-path fallback for header auth, presigned URLs, HTTP/2 authority handling, and streaming-body seed signatures. Cherry-picking #10 now would overlap with the same files and move us to a narrower preserve = globally approach, so the current mainline solution is the one to keep. Thanks again for the report and patch.