refactor: fees: split fee rate format from fee estimate mode

- Introduce a `FeeRateFormat` enum and change `CFeeRate::ToString()`
   to use it for `BTC/kvB` vs `sat/vB` output formatting.
 - Handle all enum values, hence remove default case in `CFeeRate::ToString()`
   and `assert(False)` when a `FeeRateFormat` value is not handled.
 - Keep `FeeEstimateMode` focused on fee estimation behavior by removing fee rate format
   values from `FeeEstimateMode`.
 - Update all formatting call sites and tests to pass `FeeRateFormat` explicitly, separating fee rate format
   from fee-estimation mode selection.

src/common/messages.cpp

@@ -4,11 +4,11 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <common/messages.h>
-
 #include <common/types.h>
-#include <policy/fees/block_policy_estimator.h>
 #include <node/types.h>
+#include <policy/fees/block_policy_estimator.h>
 #include <tinyformat.h>
+#include <util/fees.h>
 #include <util/strencodings.h>
 #include <util/string.h>
 #include <util/translation.h>
@@ -68,7 +68,6 @@ std::string FeeModeInfo(const std::pair<std::string, FeeEstimateMode>& mode, std
                    "less responsive to short-term drops in the prevailing fee market. This mode\n"
                    "potentially returns a higher fee rate estimate.\n", mode.first);
         default:
-            // Other modes apart from the ones handled are fee rate units; they should not be clarified.
             assert(false);
     }
 }