_messages_to_openviking_batch's pre-scan already parses and caches each
tool call's arguments into tool_calls_by_id. The pending-tool-call branch
re-parsed them via _tool_call_input(), a second parse and a second source
of truth. Reuse the cached tool_input when the id was cached (non-empty),
falling back to a parse only for the uncached empty-id case so arguments
are never dropped. No behavior change.
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