Add functional coverage for the getblockfilter error raised when a node has a known block header, but the block has never been connected.
This gap in test coverage was identified by a coverage report generated while exploring BIP 157/158. The branch in src/rpc/blockchain.cpp was never executed in the measured tests:
https://github.com/bitcoin/bitcoin/blob/64cf455b481d36182957bdcfcae7c411a7879897/src/rpc/blockchain.cpp#L3054-L3056
The existing getblockfilter functional test checks unknown block hashes and filter retrieval for active and stale blocks, but doesn’t check a known header whose block has never been connected.
This change submits only the block header of a newly mined block to a node using submitheader and verifies that in this expected state, getblockfilter returns error -5 with "Filter not found. Block was not connected to active chain." This adds coverage for the RPC’s distinction between an unknown block and a known block for which no filter has been constructed.
rpc_getblockfilter.py passes, and changing the error message makes the new assertion fail as expected.
