Merge bitcoin/bitcoin#34181: refactor: [p2p] Make ProcessMessage private again, Use references when non-null

fa43897c1d doc: Fix LLM nits in net_processing.cpp (MarcoFalke)
bbbba0fd4b scripted-diff: Use references when nullptr is not possible (MarcoFalke)
fac5415466 refactor: Separate peer/maybe_peer in ProcessMessages and SendMessages (MarcoFalke)
fac529188e refactor: Pass Peer& to ProcessMessage (MarcoFalke)
fa376095a0 refactor: Pass CNode& to ProcessMessages and SendMessages (MarcoFalke)
fada838014 refactor: Make ProcessMessage private again (MarcoFalke)
fa80cd3cee test: [refactor] Avoid calling private ProcessMessage() function (MarcoFalke)

Pull request description:

  There is a single unit test, which calls the internal `ProcessMessage` function. This is problematic, because it makes future changes harder, since they will need to carry over this public internal interface each time.

  Also, there is a mixed use of pointers and references in p2p code, where just based on context, a pointer may sometimes assumed to be null, or non-null. This is confusing when reading the code, or making or reading future changes.

  Fix both issues in a series of commits, to:

  * refactor the single unit test to call higher-level functions
  * Make `ProcessMessage` private again
  * Use references instead of implicit non-null pointers, mostly in a scripted-diff

ACKs for top commit:
  optout21:
    reACK fa43897c1d
  ajtowns:
    ACK fa43897c1d
  Crypt-iQ:
    crACK fa43897c1d
  achow101:
    ACK fa43897c1d

Tree-SHA512: d03d8ea35490a995f121be3d2f3e4a22d1aadfeab30bc42c4f8383dab0e6e27046260e792d9e5a94faa6777490ba036e39c71c50611a38f70b90e3a01f002c9e

src/test/fuzz/process_message.cpp

@@ -123,7 +123,7 @@ FUZZ_TARGET(process_message, .init = initialize_process_message)
             more_work = connman.ProcessMessagesOnce(p2p_node);
         } catch (const std::ios_base::failure&) {
         }
-        node.peerman->SendMessages(&p2p_node);
+        node.peerman->SendMessages(p2p_node);
     }
     node.validation_signals->SyncWithValidationInterfaceQueue();
     node.connman->StopNodes();