refactor(core): split file system path rules

[haasonsaas]

Summary

• move managed-file path resolution, filename validation, supported extension lists, and read-memory preview helpers into crates/browser-use-core/src/file_system/paths.rs
• preserve existing internal/test-facing re-exports for resolve_file_action_path and supported_text_extensions
• reduce crates/browser-use-core/src/file_system.rs from 1,173 lines on main to 840 lines

Validation

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