Completes the stringly-error conversion: 341 remaining
Result<_, String> signatures drop to two documented boundary sites
(a String-compat wrapper consumed across a crate boundary and a
test-only diagnostic helper). Eighty-plus enums follow the
established recipe — one enum per failure domain in a sibling
module, byte-identical Display text, From impls replacing map_err
adapters. mcp_live is fully typed (the screenshot channel included)
and all six temporary String bridges are deleted. The flaky
cli-model-discovery trio is made load-proof: exec'd sleeps so the
probe's kill actually closes the pipes, a serializing test lock,
and an escalating budget with a deadline-boundedness assertion.
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