fix(search_code): error on invalid regex instead of silent empty (#283)

With regex=true, a syntactically invalid pattern (e.g. unclosed group) made
the underlying grep fail, which handle_search_code reported as an empty result
set — indistinguishable from a legitimate no-match. Validate the user pattern
with cbm_regcomp up front and return an explicit "invalid regex" error so
callers can tell a broken pattern from zero matches. regex=false is unaffected.