refactor: Drastically narrow scope of http_bitcoin namespace and rename it to bitcoin_http

http_bitcoin was mostly used during #35182 to distinguish from http_libevent counterpart:
- The http_libevent namespace was introduced around the legacy code in 89c54ae4cb.
- The http_bitcoin namespace was introduced in 68b5d289d1 and extended in subsequent commits.
- The http_libevent namespace together with code it contained was removed in 8c1eea0777.

bitcoin_http is a better name as it is Bitcoin Core's implementation of the HTTP protocol, not HTTP protocol's implementation of bitcoin 402 payment required codes or anything like that.
The namespace only remains for a few constants and a type which don't have HTTP in their names.

src/httpserver.h

@@ -49,11 +49,10 @@ enum class HTTPRequestMethod {
     PUT
 };
 
-namespace http_bitcoin {
-    class HTTPRequest;
-}
+class HTTPRequest;
+
 /** Handler for requests to a certain HTTP path */
-using HTTPRequestHandler = std::function<void(http_bitcoin::HTTPRequest* req, const std::string&)>;
+using HTTPRequestHandler = std::function<void(HTTPRequest* req, const std::string&)>;
 
 /** Register handler for prefix.
  * If multiple handlers match a prefix, the first-registered one will
@@ -63,9 +62,7 @@ void RegisterHTTPHandler(const std::string &prefix, bool exactMatch, const HTTPR
 /** Unregister handler for prefix */
 void UnregisterHTTPHandler(const std::string &prefix, bool exactMatch);
 
-namespace http_bitcoin {
-using util::LineReader;
-
+namespace bitcoin_http {
 //! Shortest valid request line, used by libevent in evhttp_parse_request_line()
 inline constexpr size_t MIN_REQUEST_LINE_LENGTH = std::string_view("GET / HTTP/1.0").size();
 
@@ -83,6 +80,7 @@ inline constexpr uint64_t MAX_BODY_SIZE{32_MiB};
 struct ContentTooLargeError : std::runtime_error {
     using std::runtime_error::runtime_error;
 };
+} // namespace bitcoin_http
 
 class HTTPHeaders
 {
@@ -163,9 +161,9 @@ public:
      * @throws      std::runtime_error if data is invalid.
      */
     /// @{
-    bool LoadControlData(LineReader& reader);
-    bool LoadHeaders(LineReader& reader);
-    bool LoadBody(LineReader& reader);
+    bool LoadControlData(util::LineReader& reader);
+    bool LoadHeaders(util::LineReader& reader);
+    bool LoadBody(util::LineReader& reader);
     /// @}
 
     void WriteReply(HTTPStatusCode status, std::span<const std::byte> reply_body = {});
@@ -630,6 +628,5 @@ void InterruptHTTPServer();
 
 /** Stop HTTP server */
 void StopHTTPServer();
-} // namespace http_bitcoin
 
 #endif // BITCOIN_HTTPSERVER_H