Strip entity headers on POST-to-GET redirects

[ryanfowler]

Summary

• Track when a redirect converts a request to a bodyless GET and strip entity-specific headers on the next attempt
• Remove Content-Type, Content-Length, and Transfer-Encoding before signing and building redirected requests
• Preserve the same sanitization for digest retries after such redirects
• Add regression coverage for the redirect path and the affected header set

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features
• Added focused unit and integration tests for POST-to-GET redirect header stripping