Reject only the unroutable partitions in a ProduceSync call

[koloss2001]

Summary

Port of #99 from release-0.1 to main.

• Named missing leaders already fall back to a live agent. Remaining routing misses, such as Leader < 0, unknown topics, topic-level metadata errors, or an empty pool, now reject only the affected records in ProduceSync. The call waits for accepted records to finish, without waiting on Metadata. Produce is unchanged.
• Repeated record pointers receive the correct outcome at every input position, including success, routing rejection, cancellation, and terminal failure. Additional result copying runs only when pointers repeat.
• Repeat Back off on-demand Metadata refreshes while a leader stays excluded (release-0.1) #97's periodic-Close regression scenario 40 times to reliably detect a missing refreshCtx.Err() check; a single run can pass about half the time without it.