[Bug] Missing fields in `getblockheader` rpc

[qlrd]

Describe the bug
While refactoring a test for getblockheader JSONrpc test, it was noted that floresta response has missing fields in response, namely mediantime, target, chainwork and nTx.

Steps to Reproduce

Start a florestad and a bitcoind, both in regtest and then call:

$> BLOCKHASH=$(floresta-cli --network regtest getblockhash 0)
$> floresta-cli --network regtest getblockheader $BLOCKHASH
$> bitcoin-cli -network regtest getblockheader $BLOCKHASH

Expected behavior

floresta-cli should comply with Bitcoin-Core and return a json like:

--- floresta.json
+++ bitcoin-core.json
 {
   'hash': '0f9188f13cb7b2c71f2a335e3a4fc328bf5beb436012afca590b1a11466e2206',
   'confirmations': 11,
   'height': 0,
   'version': 1,
   'versionHex': '00000001',
   'merkleroot': '4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b',
   'time': 1296688602,
+  'mediantime': 1296688602,
   'nonce': 2,
   'bits': '207fffff',
+  'target': '7fffff0000000000000000000000000000000000000000000000000000000000',
-  'difficulty': 1,
+  'difficulty': 4.656542373906925e-10,
+  'chainwork': '0000000000000000000000000000000000000000000000000000000000000002',
+  'nTx': 1,
-  'previousblockhash': '0000000000000000000000000000000000000000000000000000000000000000',
   'nextblockhash': '369bec98e6b8ad7b2a46c20374823d75cf0792e6e8ef9287bfc256c3021de87f'
 }

Build environment

version v0.8.0-20-geafa9d1-dirty compiled for aarch64-macos- with rustc 1.90.0-nightly (ba7e63b63 2025-07-29)

Additional context
Related with #590

[Vedd-Patel]

analysing this issue for work!

[Vedd-Patel]

here's how(what) i'll tackle this one-

missing fields to add

• mediantime - median of the last 11 block time stamps(for this one we need to walk back till 11 ancestors of the cuurent node)
• target - expand the compact bits field into the fully 32byte hex target
• chainwork - cumulative chain work up to this block(this shoyld be already be stored in index of block, so we just need ti expose this)
• nTx - no of txns in the block

difficulty value also wrong

• floresta returns 1 while core returns the actual calculated difficulty- the calculation needs to be fixed alongside the above mentioned points

previousblockhash behavior

• core omits this field for the genesis block rather than returning the zero hash