rpc: Set best header in reconsiderblock #30652

The reconsiderblock RPC resets the blocks flags and then calls ActivateBestChain(). But this doesn't set m_best_header which leaves us in an inconsistent state until something else triggers a change, like connecting a new block. But, until then, getblockchaininfo, for example, reports having less headers than blocks.

I found this while working on a more extensive tests for #29553.

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See also #26245.

It's a duplicate of #29913 (but see also my last comment there why the problem is harder than it seems)

@mzumsande thanks