mining: preserve lookup result count without a mempool #36132

1. 
   l0rinc commented at 4:11 PM on August 31, 2026: contributor

   Problem: The mining interface returns one nullable transaction for every requested txid or wtxid so callers can match results by position. When its NodeContext has no mempool (not sure that's possible, but there's a condition for it which violates the contract), both lookup methods return an empty vector for every nonempty request and discard that positional mapping, see: https://github.com/bitcoin/bitcoin/blob/7b6f9ba7bad13b0c4169259000f7802854cdda0d/src/interfaces/mining.h#L202-L203

   Fix: Return a vector sized to the request when no mempool is available. Default-initialized null entries match the existing representation for identifiers absent from an available mempool, so callers receive one response position per request in either state.

2. test: characterize mining lookups without mempool 14a8d963f4
3. mining: preserve transaction lookup result count

   The mining interface returns one nullable transaction for every requested txid or wtxid so callers can match results by position.
   When a `NodeContext` has no mempool, both lookup methods return an empty vector and discard that positional mapping.
   
   Return a vector sized to the request in this state. Its default-initialized entries represent missing transactions, matching
   the existing result for identifiers absent from an available mempool.

   ac2c6c4ef1
4. DrahtBot added the label Mining on Aug 31, 2026
5. 
   DrahtBot commented at 4:11 PM on August 31, 2026: contributor

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   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36132.

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6. sedited requested review from Sjors on Aug 31, 2026

Review Requested
Sjors