fix: copy SearchConfig before mutating .limit to prevent shared-state clobber

[totto]

What

search_recall() assigned one of the module-level SearchConfig constants directly to a local variable and then mutated .limit on it:

search_config = EDGE_HYBRID_SEARCH_RRF  # reference, not copy
search_config.limit = num_results        # mutates the global constant

Why it matters

This is a concurrency hazard. Every concurrent call to search_recall() races to set limit on the same shared object. One request can silently cap another request's result count to whatever was set most recently.

Even in a single-threaded context, the module-level constant is permanently modified after the first call — subsequent calls that rely on the default limit will get the last caller's value instead.

Fix

Call .model_copy(deep=True) before mutation so each invocation works on its own independent copy:

search_config = (
    EDGE_HYBRID_SEARCH_RRF if center_node_uuid is None else EDGE_HYBRID_SEARCH_NODE_DISTANCE
).model_copy(deep=True)
search_config.limit = num_results

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[totto]

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