Fix tcp_server::bind() to use non-throwing listen()

tcp_server::bind() could throw because tcp_acceptor::listen() had
throwing overloads. This changes listen() to return [[nodiscard]]
std::error_code, allowing bind() to properly propagate errors without
throwing.

API change for tcp_acceptor::listen():
  Before: void listen(endpoint, int backlog = 128)        // throws
          void listen(endpoint, std::error_code&)         // out-param
          void listen(endpoint, int, std::error_code&)    // out-param

  After:  [[nodiscard]] std::error_code listen(endpoint, int backlog = 128)

The [[nodiscard]] attribute ensures callers cannot accidentally ignore
errors. This aligns with signal_set and socket options which already
use non-throwing return values.

Changes:
- Fix tcp_server::bind() to capture and return listen() error code
- Consolidate tcp_acceptor::listen() to single non-throwing overload
- Document error conditions (address_in_use, address_not_available,
  permission_denied, operation_not_supported) and @throws Nothing
- Update test utilities (socket_pair, mocket) with error checks
- Update all unit tests (~20 call sites) to check return values
- Update documentation (tcp_acceptor.adoc, tls.adoc, signals.adoc,
  endpoints.adoc) with new error-handling pattern

Author: MungoG

doc/modules/ROOT/pages/guide/endpoints.adoc

@@ -202,7 +202,8 @@ auto [ec] = co_await s.connect(target);
 [source,cpp]
 ----
 corosio::tcp_acceptor acc(ioc);
-acc.listen(corosio::endpoint(8080));  // Bind to all interfaces
+if (auto ec = acc.listen(corosio::endpoint(8080)))  // Bind to all interfaces
+    return ec;
 ----
 
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