[v2.0] Phase 3: Python wrapper with graceful fallback

[fsecada01]

Implement TextSpitter/splitters.py — thin Python classes that use the Rust backend when available and fall back to pure Python transparently.

Wiki: Phase 3 detail
Branch: feature/rust-backend

Tasks

• 3.1 Implement _RUST_AVAILABLE import guard (try: from text_spitter_rust import ...; except ImportError)
• 3.2 Implement CharacterTextSplitter Python class with split_text / split_texts routing to Rust or Python backend
• 3.3 Implement _split_text_python — fully working pure-Python fallback (not a stub); must produce identical output to Rust implementation
• 3.4 Expose use_rust=False parameter for forcing pure-Python mode (debugging + parity testing)

Acceptance criteria

• CharacterTextSplitter(use_rust=True).split_text(text) == CharacterTextSplitter(use_rust=False).split_text(text) for all inputs
• If text_spitter_rust is not installed, the class works silently via Python fallback