rpc: handle createNewBlock() returning nullptr during shutdown #36047

Problem

createNewBlock() returns nullptr only when the node is shutting down (WaitTipChanged returns nullopt on interrupt and nothing else). The mining RPCs wrapped the result in CHECK_NONFATAL, reporting an expected shutdown condition as "Internal bug detected".

Fix

getblocktemplate and generateblock now throw "Shutting down" (RPC_CLIENT_NOT_CONNECTED), matching how getblocktemplate's longpoll path already reports the same condition. generateBlocks stops mining and returns the blocks found so far, matching its existing interrupt handling.

Testing

Add regression coverage: a longpoll getblocktemplate interrupted by shutdown must error with "Shutting down". Without the fix this raced ~50/50 between the correct error and the internal-bug error, because the IsRPCRunning() check runs before RPC is disabled during shutdown.

Fixes #34262

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36047.

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