feat: port regex filter plugin to cpex

[lucarlig]

Summary

• add a new Rust-owned regex_filter plugin package under plugins/rust/python-package
• port the regex replacement engine and gateway hook integration from mcp-context-forge branch 2996_rewrite_regex_filter_rust
• port plugin-local Rust and Python tests for regex behavior and hook handling

Verification

• cargo test
• make check-all
• uv run pytest tests/ -v
• uv run pytest tests/ -q

Notes

• This is a draft PR as requested.
• The standard detailed-code-review workflow has not been run yet in this task and should be run before review/merge.