refactor(tools): move session tools to the agent_tools registry (#4454)

Moves create_session, list_sessions, send_to_session and manage_session out of
ai_interaction.py into src/agent_tools/session_tools.py (the do_ prefix
dropped) and registers them in TOOL_HANDLERS, so dispatch flows through the
registry instead of the dispatch_ai_tool elif in tool_execution.py. Same
pattern as the model-interaction move.

The bodies move verbatim; each fetches the runtime-set session manager via a
get_session_manager() shim, and reuses _resolve_model / AI_CHAT_TIMEOUT from
ai_interaction. manage_session's internal 'list' alias is repointed from the
old do_list_sessions to the moved list_sessions. stream_ai_tool (dead, no
callers) and do_pipeline stay put. dispatch_ai_tool loses its four now-unused
branches.

Tests: test_session_tools_registry covers registration, owner threading, the
manage_session->list_sessions delegation, graceful no-manager handling, and
registry dispatch. Verified end-to-end against a live SessionManager.

src/agent_tools/__init__.py

@@ -24,6 +24,7 @@ from .filesystem_tools import ReadFileTool, WriteFileTool, EditFileTool, LsTool,
 from .document_tools import CreateDocumentTool, UpdateDocumentTool, EditDocumentTool, SuggestDocumentTool, ManageDocumentTool
 from .model_interaction_tools import ChatWithModelTool, AskTeacherTool, ListModelsTool
 from .bg_job_tools import ManageBgJobsTool
+from .session_tools import CreateSessionTool, ListSessionsTool, SendToSessionTool, ManageSessionTool
 
 TOOL_HANDLERS = {
     "bash": BashTool().execute,
@@ -46,6 +47,10 @@ TOOL_HANDLERS = {
     "ask_teacher": AskTeacherTool().execute,
     "list_models": ListModelsTool().execute,
     "manage_bg_jobs": ManageBgJobsTool().execute,
+    "create_session": CreateSessionTool().execute,
+    "list_sessions": ListSessionsTool().execute,
+    "send_to_session": SendToSessionTool().execute,
+    "manage_session": ManageSessionTool().execute,
 }
 
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