fix: honor default mask strategy in rust pii filter

[lucarlig]

Summary

This fixes the Rust pii_filter package so built-in PII patterns honor default_mask_strategy instead of forcing hardcoded mask strategies.

Root Cause

Built-in pattern definitions in plugins/rust/python-package/pii_filter/src/patterns.rs stored explicit mask_strategy values, so detection never fell back to config.default_mask_strategy.

What Changed

• Changed built-in Rust pattern definitions to inherit default_mask_strategy
• Kept explicit mask-strategy overrides for custom patterns only
• Updated Rust regression tests to verify global fallback behavior and masked output

Closes #22
Related to IBM/mcp-context-forge#3820