Bugfix: Miner: Don't reuse block_reserved_weight for "block is full enough to give up" weight delta

PR bitcoin#30356 incorrectly changed a constant of `4000` to `m_options.coinbase_max_additional_weight` in the check for when to give up finding another transaction to fill the block:

```diff
             if (nConsecutiveFailed > MAX_CONSECUTIVE_FAILURES && nBlockWeight >
-                    m_options.nBlockMaxWeight - 4000) {
+                    m_options.nBlockMaxWeight - m_options.block_reserved_weight) {
                 // Give up if we're close to full and haven't succeeded in a while
                 break;
             }
```

But this constant did not deal with the reserved weight at all. It was in fact simply checking if the block was close to full, and if so, giving up finding another transaction to pad it with after `MAX_CONSECUTIVE_FAILURES` failed attempts.

It doesn't seem very logical to reuse the reserve weight for this purpose, and it would be overcomplicated to add yet another setting, so this PR changes it to a new constexpr.

Author: luke-jr

src/node/miner.cpp

@@ -314,6 +314,7 @@ void BlockAssembler::addPackageTxs(int& nPackagesSelected, int& nDescendantsUpda
     // close to full; this is just a simple heuristic to finish quickly if the
     // mempool has a lot of entries.
     const int64_t MAX_CONSECUTIVE_FAILURES = 1000;
+    constexpr int32_t BLOCK_FULL_ENOUGH_WEIGHT_DELTA = 4000;
     int64_t nConsecutiveFailed = 0;
 
     while (mi != mempool.mapTx.get<ancestor_score>().end() || !mapModifiedTx.empty()) {
@@ -395,7 +396,7 @@ void BlockAssembler::addPackageTxs(int& nPackagesSelected, int& nDescendantsUpda
             ++nConsecutiveFailed;
 
             if (nConsecutiveFailed > MAX_CONSECUTIVE_FAILURES && nBlockWeight >
-                    m_options.nBlockMaxWeight - m_options.block_reserved_weight) {
+                    m_options.nBlockMaxWeight - BLOCK_FULL_ENOUGH_WEIGHT_DELTA) {
                 // Give up if we're close to full and haven't succeeded in a while
                 break;
             }