[rust] Add SnapshotId type alias

[Hashim1999164]

Summary

• add a public SnapshotId alias alongside the existing table, partition, and bucket ID aliases
• use it across snapshot metadata, admin APIs, RPC constructors, and the Python binding
• keep protobuf and C++ FFI boundaries on their concrete wire types

This is a readability and consistency change only; SnapshotId remains an alias for i64.

Fixes #636.

Validation

• cargo fmt --all -- --check
• cargo test -p fluss-rs --lib --all-features (572 passed)
• cargo check -p fluss_python
• cargo clippy -p fluss-rs --lib --all-features (completed with two pre-existing for_kv_map warnings in accumulator.rs)

The stricter all-targets Clippy command with -D warnings stops on those same unrelated warnings.

AI assistance

AI assistance was used to identify the remaining hand-written snapshot ID types and draft the mechanical alias substitutions. I reviewed the complete diff and ran the validation above.