refactor(lsp-manager): replace handler architecture with response registry and modular providers

* Remove legacy handlers system (BaseHandler, DefaultHandler, JavaHandler, PythonHandler, HandlerRegistry)
* Introduce response_handlers module with ResponseRegistry for LSP response routing
* Replace HandlerRegistry usage in LSPManager with ResponseRegistry
* Convert LSPManagerUI client lifecycle to GObject signals
* Remove GLib.idle_add usage in LSP client event dispatch
* Move completion request logic into LSPServerEventsMixin
* Replace provider.py and provider_response_cache.py with modular provider/ package
* Simplify plugin wiring via response_registry.set_event_hub()

This refactor decouples response handling, simplifies event flow, and prepares
the LSP manager for easier language-specific extensions.

plugins/code/ui/lsp_manager/response_handlers/base.py

@@ -0,0 +1,31 @@
+# Python imports
+from abc import ABC
+
+# Lib imports
+
+# Application imports
+
+
+
+class BaseHandler:
+    def __init__(self):
+        self.context        = None
+        self.response_cache = None
+
+
+    def set_context(self, context):
+        self.context = context
+
+    def set_response_cache(self, response_cache):
+        self.response_cache = response_cache
+
+    @property
+    def emit(self):
+        return self.context.emit
+
+    @property
+    def emit_to(self):
+        return self.context.emit_to
+
+    def handle(self, method: str, response, controller):
+        pass