Test the fmm engine guards, string weights, and config key completions - #609

Three behaviors had no test at all in the fast tier. Asking for the
deprecated "fmm"/"fast_matrix_market" Matrix Market engine when
fast_matrix_market is not installed must warn about the deprecation
before it fails, and then fail with an ImportError naming the engine;
neither read nor write had coverage for that, since the existing engine
tests skip themselves when the package is missing. String edge weights
in from_networkx infer a 1-D <U array, so only the dtype-kind half of
the guard sends them down the scipy fallback; the neighboring
array-weight test exercises only the shape half. And the config
mapping's ipython_key_completions hook, which drives config[<tab>]
in IPython, was never called.

Each test was checked by perturbing the line it covers and confirming
it goes red.

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