fix: handle non-capturing groups (?:...) in JSON schema pattern converter

[XciD]

Summary

• Fix SIGSEGV in _visit_pattern() when a JSON schema pattern contains non-capturing groups (?:...)
• Skip ?: after ( to treat non-capturing groups as regular groups
• Safely handle unsupported lookahead/lookbehind syntax by skipping to matching )

Root cause

In common/json-schema-to-grammar.cpp, _visit_pattern() line ~420: when the parser sees ( followed by ?, it pushes a warning but does not advance i past ?:. The recursive transform() call then interprets ? as a quantifier and calls seq.back() on an empty vector (undefined behavior, SIGSEGV).

Reproducer

Start llama-server with any model, then:

curl -X POST http://127.0.0.1:8080/v1/messages \
  -H "Content-Type: application/json" \
  -H "x-api-key: dummy" \
  -H "anthropic-version: 2023-06-01" \
  -d '{"model":"any","max_tokens":50,"messages":[{"role":"user","content":"hi"}],
  "tools":[{"name":"t","description":"t","input_schema":{"type":"object",
  "properties":{"arr":{"type":"array","items":{"anyOf":[{"type":"object",
  "properties":{"d":{"type":"string","pattern":"^(?:foo)$"}},
  "required":["d"]}]}}}}}],"tool_choice":{"type":"auto"}}'

The server crashes with SIGSEGV before returning a response. With this fix, it returns HTTP 200 correctly.

This affects real-world clients (e.g. Claude Code with Notion MCP tools) that send tool schemas containing date validation patterns like ^(?:(?:\d\d[2468][048]|...)-02-29|...)$.

[aldehir]

Can you add a test to tests/test-json-schema-to-grammar.cpp?