test: code_symbol_search touched_files order flipped after #2271/#2272

[ScriptedAlchemy]

After rebasing onto 56839b9 (master with #2271 and #2272), cargo test -p tracedecay --features test-transport,test-helpers --test mcp_suite -- graph_query_test::typed_evidence_trailers::callable_code_reads_carry_their_files_and_the_accounting_footer fails:

typed_evidence_trailers.rs:369:5
  left: Array [String("src/lib.rs"), String("src/walk.rs")]
 right: Array [String("src/walk.rs"), String("src/lib.rs")]

tracedecay_code_symbol_search with meta.order = "source_position" now reports touched_files in a different order. The same test passed on 14caed2. Found while landing #2273, which does not touch this read path.