Support filter chains on model listeners

[adilhafeez]

Summary

• Adds filter_chain support on model-type listeners, allowing input guardrails on direct LLM proxy requests without an agent layer
• Reuses the existing PipelineProcessor.process_filter_chain() subsystem from agent listeners
• Validates filter_chain IDs in Python config generator

Closes #729

Changes

• Config schema: Added filter_chain (array of strings) to listener object
• Rust Listener struct: Added filter_chain: Option<Vec<String>> field
• LLM handler: After request parsing, checks for model listener filter chain, runs PipelineProcessor.process_filter_chain(), updates messages or returns error
• Config validation: Validates filter_chain IDs reference valid agent/filter IDs
• Reference config + tests: Updated for new field

Test plan

• cargo test --lib — 294 passed
• cargo clippy -- -D warnings — clean
• WASM build (wasm32-wasip1) — successful
• uv run pytest -v — 34 passed