feat: expose state metadata in Python API

[xuzha]

Summary

• Add optional state_metadata support to Python Context.add and add_many.
• Convert Python metadata mappings into the existing Rust StateMetadata fields.
• Add round-trip tests for full and partial state metadata writes.

Testing

• uv run --extra tests pytest tests/test_search.py tests/test_state_metadata.py
• uv run --extra tests ruff check python tests
• uv run --extra dev pyright
• RUSTC=/Users/xuzhang6/Library/Caches/puccinialin/rustup/toolchains/stable-aarch64-apple-darwin/bin/rustc /Users/xuzhang6/Library/Caches/puccinialin/rustup/toolchains/stable-aarch64-apple-darwin/bin/cargo test --manifest-path python/Cargo.toml

Notes

• Full uv run --extra tests pytest currently has existing failures in compaction/persistence/S3 tests unrelated to this change.
• cargo fmt could not be run locally because the available cached Rust toolchain does not include rustfmt.