[client] Allow bootstrap endpoint with unknown server type

[fxbing]

Purpose

Linked issue: close #621

Allow the Rust client to bootstrap from either coordinator or tablet server endpoints after server type validation is enabled.

Brief change log

• Add ServerType::Unknown for bootstrap seed nodes.
• Use ServerType::Unknown when creating the initial bootstrap ServerNode.
• Skip server type validation only when the expected type is Unknown.
• Add unit coverage for unknown expected server type.
• Update Elixir binding server type mapping for the new variant.

Tests

• cargo fmt --all -- --check
• PROTOC=/opt/homebrew/bin/protoc cargo test -p fluss-rs --lib server_type_validation
• PROTOC=/opt/homebrew/bin/protoc cargo test -p fluss-rs --lib
• PROTOC=/opt/homebrew/bin/protoc cargo clippy --all-targets --workspace -- -D warnings

API and Format

This adds a public ServerType::Unknown enum variant. It does not change the wire protocol or storage format.

Documentation

No user-facing documentation change.

[leekeiabstraction]

Thank you for your first contribution and improving the client! Left a question.

[leekeiabstraction]

Should a simple integration test case be made in existing IT testing with unknown type/bootstrapping against tablet server?