The salvaged fix gave `_find_missing` and `_flatten_messages` each their own
visited-set loop, next to the one `_is_session_expired_error` already had —
three copies of the same idiom in one module. Collapse them into
`_iter_exception_nodes` (pre-order, left-to-right, each node once, bounded by
`_EXC_TRAVERSAL_MAX_NODES`) and read all three scans off that list. Acyclic
output is byte-identical: the missing-executable search keeps its depth-first
order and a message-less leaf still renders as its class name.
Tests move from the issue-numbered file into `tests/tools/test_mcp_tool_errors.py`
(mirror of the source module): a two-node cycle renders the real messages, and a
missing stdio binary wrapped deeper than the recursion limit with the chain
looping back to the top is still reported as the missing executable. Both are
red on origin/main (RecursionError).
Co-authored-by: Stephan Mongstad <stephan@users.noreply.github.com>
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