The `if (error instanceof WebError) throw error` guard is dead code in the
exa and perplexity providers: their mappers (mapExaResponse /
mapPerplexityResponse) never throw a WebError — a wrong-shape body throws a
TypeError, which the catch correctly translates to WEB_PROVIDER_ERROR. The
guard was added for symmetry with the deepseek provider, whose mapper DOES
throw a WebError in strict mode (no web_search_tool_result block), so it
keeps the rethrow. The unreachable lines tripped the per-file 100% coverage
gate.
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