Split agent runtime helpers

[haasonsaas]

Summary

• move agent LLM invocation/fallback/usage helpers into crates/browser-use-core/src/agent/llm.rs
• move step metadata and screenshot-decision helpers into crates/browser-use-core/src/agent/run_state.rs
• reduce crates/browser-use-core/src/agent.rs from 1,073 to 971 lines

Test Plan

• cargo check -p browser-use-core
• cargo test -p browser-use-core --lib
• cargo fmt --check
• git diff --check