h3: support trailer field sections

[SC123667]

Summary

• add explicit Rust and C APIs for additional HTTP/3 field sections, covering request and response trailers as well as informational response chaining
• track send and receive message phases so initial DATA, duplicate trailers, and HEADERS or DATA after trailers are rejected with H3_FRAME_UNEXPECTED
• QPACK-decode every received field section and surface trailers through the existing Headers event/callback with clarified fin semantics
• preserve atomic HEADERS writes and add a flow-control retry path for blocked trailer sections

Standards

• RFC 9114 Section 4.1 message sequencing
• RFC 9204 field-section decoding through the existing QPACK path

Testing

• cargo fmt --all -- --check
• typos -c ./typos.toml
• cargo clippy --all -- -D warnings
• cargo clippy --all -F ffi -- -D warnings
• cargo build --all -F ffi --verbose
• cargo test --verbose (563 unit tests and 1 doc test passed)
• cargo build --all --no-default-features -F h3 and cargo test --no-default-features -F h3 (558 unit tests and 1 doc test passed)
• cargo build --all --no-default-features -F ffi,h3 and cargo test --no-default-features -F ffi,h3 (558 unit tests and 1 doc test passed)
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --all-features
• generated-header comparison, C/C++ header syntax checks, and exported-symbol verification

Resolves #226