release: prepare a3s-code 6.4.0

[ZhiXiao-Lin]

Summary

• add structured web-search provider failure metadata and bounded fallback across built-in engines
• restore DuckDuckGo and Wikipedia as the built-in defaults while keeping AnySearch explicit
• accept a complete schema-valid streamed object when a compatible provider omits or delays Done
• publish the Code core and SDK manifests as 6.4.0
• require a3s-search 2.1.0 and a3s-flow 0.4.3

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --lib --bins -- -D warnings
• cargo test --workspace (2539 passed; 14 ignored)
• cargo test --workspace --all-features --lib (2585 passed; 14 ignored)
• public API SemVer check against 5.3.5
• Node and Python SDK protocol/API alignment and builds
• deterministic agent convergence benchmark (4/4)

Dependencies

This PR remains draft until A3S Search 2.1.0 and A3S Flow 0.4.3 are published to crates.io.