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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Reviews

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #35675 (mining: add block template manager by ismaelsadeeq)
• #32468 (rpc: generateblock to allow multiple outputs by polespinasa)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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