Refactor lsp.Server God Object: extract chat and completion subsystems

The Server type accumulated two large, tangled feature subsystems (in-editor
chat and code completion) alongside its core LSP dispatch role. Pull them into
cohesive types that own their state and logic while delegating shared
infrastructure back to Server via a back-reference.

- Add completionService (completion_service.go): owns the completion
  cache/throttle state (completionState) and all completion request-handling
  logic (handleCompletion, plan/jobs/execute, provider-native path,
  post-processing, message building, prefix heuristics). Methods moved off
  Server in handlers_completion.go to completionService.
- Add chatService (chat_service.go + chat_handlers.go): owns the input-activity
  clock (lastInput, with its own mutex instead of Server.mu) and all in-editor
  chat logic (detection, transcript history, message building, edit
  application, inline prompts) plus the slash-command handlers
  (chat_commands.go).
- Server now holds chat/completion fields, wires them in NewServer, and
  delegates (dispatch table, didChange, debounce gate) to them. Thin Server
  shims preserve the existing completion-state API for callers/tests.

Pure refactor: no behavior or LSP protocol changes. Tests adjusted only to
reach the relocated methods/state via the services. All tests pass with -race;
coverage 86.1%.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/internal/lsp/document_test.go b/internal/lsp/document_test.go
index e6fba40..849346d 100644
--- a/internal/lsp/document_test.go
+++ b/internal/lsp/document_test.go
@@ -38,15 +38,17 @@ func newTestServer() *Server {
 			PromptCodeActionGoTestUser:        "Function under test:\n{{function}}",
 		},
 	}
-	return &Server{
+	s := &Server{
 		logger: log.New(io.Discard, "", 0),
 		docs:   make(map[string]*document),
 		cfg:    cfg,
 		codeActionSubsystem: codeActionSubsystem{
 			llmClientRegistry: llmClientRegistry{llmProvider: llmutils.CanonicalProvider(cfg.Provider)},
 		},
-		completionSubsystem: completionSubsystem{completionState: completionState{}},
 	}
+	s.chat = newChatService(s)
+	s.completion = newCompletionService(s)
+	return s
 }
 
 func initServerDefaults(s *Server) {