Merge bitcoin/bitcoin#26261: p2p: cleanup LookupIntern, Lookup and LookupHost

5c832c3820 p2p, refactor: return `std::optional<CNetAddr>` in `LookupHost` (brunoerg)
34bcdfc6a6 p2p, refactor: return vector/optional<CService> in `Lookup` (brunoerg)
7799eb125b p2p, refactor: return `std::vector<CNetAddr>` in `LookupHost` (brunoerg)
5c1774a563 p2p, refactor: return `std::vector<CNetAddr>` in `LookupIntern` (brunoerg)

Pull request description:

  Continuation of #26078.

  To improve readability instead of returning a bool and passing stuff by reference, this PR changes:

  - `LookupHost` to return `std::vector<CNetAddr>`
  - `LookupHost` to return `std::optional<CNetAddr>`
  - `Lookup` to return `std::vector<CService>`
  - `Lookup` to return `std::optional<CService>`.
  - `LookupIntern` to return `std::vector<CNetAddr>`

  As discussed in #26078, it would be better to avoid using `optional` in some cases, but for specific `Lookup` and `LookupHost` functions it's necessary to use `optional` to verify if they were able to catch some data from their overloaded function.

ACKs for top commit:
  achow101:
    ACK 5c832c3820
  stickies-v:
    re-ACK 5c832c3820 - just addressing two nits, no other changes
  theStack:
    re-ACK 5c832c3820

Tree-SHA512: ea346fdc54463999646269bd600cd4a1590ef958001d2f0fc2be608ca51e1b4365efccca76dd4972b023e12fcc6e67d226608b0df7beb901bdeadd19948df840

src/qt/optionsdialog.cpp

@@ -406,9 +406,8 @@ void OptionsDialog::updateProxyValidationState()
 
 void OptionsDialog::updateDefaultProxyNets()
 {
-    CNetAddr ui_proxy_netaddr;
-    LookupHost(ui->proxyIp->text().toStdString(), ui_proxy_netaddr, /*fAllowLookup=*/false);
-    const CService ui_proxy{ui_proxy_netaddr, ui->proxyPort->text().toUShort()};
+    const std::optional<CNetAddr> ui_proxy_netaddr{LookupHost(ui->proxyIp->text().toStdString(), /*fAllowLookup=*/false)};
+    const CService ui_proxy{ui_proxy_netaddr.value_or(CNetAddr{}), ui->proxyPort->text().toUShort()};
 
     Proxy proxy;
     bool has_proxy;