Use PCRE2 for regex search when available

[Gabriel-Darbord]

Summary

• add pharo-pcre2 as an MCP baseline dependency loaded with its core group
• route search-tool regex filtering through PCRE2 when the native library is available
• fall back to Pharo's basic regex engine when the PCRE2 native library is unavailable
• keep public regex syntax errors normalized as RegexSyntaxError

Verification

• P13: loaded MCP baseline Core group
• P13: public method_metadata_search with PCRE2 lookbehind regex
• P13: MCP-Tests