trace_path resolved a function_name from the first row of an unordered name
query with no ambiguity check, so a same-named entity (e.g. a shell script's
main()) could silently shadow the intended C main(). get_code_snippet
reported "ambiguous" for a short name even when one match was the obvious
definition (the .c body vs a .h declaration).

Fix: add a deterministic resolution ranking — a callable label outranks a
module, then the larger definition by line span wins, preferring a real
definition without hardcoding file extensions — and a picker that flags a
genuine tie. trace_path now traces the preferred node and returns the
existing ambiguous-suggestions response on a true tie instead of silently
taking nodes[0]; get_code_snippet resolves directly to the preferred match,
reporting ambiguity only for real ties. Adds regression tests
tool_trace_call_path_ambiguous and tool_trace_call_path_prefers_definition.

Signed-off-by: Kris Kersey <kris@kerseyfabrications.com>