…unnel usage)

GCS server does not handle requests with port, and simply report an
error:

    <?xml version="1.0"?>
    <?xml version='1.0' encoding='UTF-8'?>
    <Error>
        <Code>InvalidURI</Code>
        <Message>Couldn't parse the specified URI.</Message>
        <Details>Invalid URL: storage.googleapis.com:443/...</Details>
    </Error>

Removing the port fixes the issue. Note that there is port in the Host
header anyway.

Note, this is a problem only for proxy in a tunnel mode, since only it
sends such requests, other sends requests directly via HTTP methods.

Replace space indentation with tabs to match project conventions.
Fix copy-paste error in setProxyProtocol() error message.
Update copyright year in ProxyConfig.h.

- Fix missing _proxySessionFactory registration in (host, port, ProxyConfig)
  constructor causing UnknownURISchemeException at connection time
- Fix HTTPSClientSession::proxyRequestPrefix() regression: return empty
  string in tunnel mode to avoid corrupting request URIs
- Fix empty proxyAuthenticate() override silently dropping credentials
  in non-tunnel HTTPS proxy mode
- Add protocol validation and connected-state check to setProxyConfig()
  and setGlobalProxyConfig()
- Wrap destructor unregisterProtocol() in try/catch to prevent
  std::terminate
- Include HTTP status code in proxyConnect() error message
- Use InvalidArgumentException instead of IllegalStateException for
  protocol validation
- Extract factory registration into initProxySessionFactory() helpers
  to prevent future missed-constructor bugs
- Modernize ProxyAuthentication to enum class with None/Basic/Digest/NTLM
- Use default member initializers in ProxyConfig (enables C++20
  designated initializers)
- Add [[nodiscard]] to key getters, modernize copy deletion to = delete
- Add 10 new unit tests covering ProxyConfig defaults, protocol
  validation, setters, request prefix, non-tunnel mode, global config,
  and bypass behavior
- Update HTTPTestServer to handle proxy requests without default port
- Fix copyright year and doc-comment style

…ADME"

This reverts commit 8684433.