Add session creation and browsing to Mason chat UI

[shivam5]

Summary

• add POST /api/session/new to create a fresh UUID session and rotate the existing routing cookie
• list recent actor-scoped managed sessions in the LangGraph chat UI, with the current local session as the in-memory fallback
• allow opening a listed managed session after verifying actor ownership, then reload its transcript, pending HITL state, and recovery state
• exclude internal durability sessions, checkpoints, heartbeats, and other runtime records from the session list and chat transcript responses
• keep the desktop chat, session rail, and demo controls independently scrollable while preserving normal page scrolling on smaller screens
• document the session APIs and add coverage for local and Databricks Apps cookie rotation

Routing behavior

The client still does not send session_id in invocation request bodies. The session id is the routing key:

• deployed Apps use __Host-databricks-app-router
• local development uses mason-local-session
• the existing TODO to move API clients to X-Routing-Key remains

Creating or opening a session replaces the routing cookie, so sticky routing and the LangGraph thread/session id stay identical.

Session Store filtering

The Session Store continues to retain LangGraph checkpoints and durability events. The browser-facing session APIs only return message-shaped items with supported chat roles, and the session picker excludes internal mason-demo-durability execution sessions.

Validation

• uv run pytest tests/unit_tests -q — 146 passed
• generated mason init --framework langgraph --enable-chat-app project tests — 27 passed, 1 skipped
• uv run ruff check src tests templates/agent-langgraph templates/ui/agent-langgraph
• uv run ruff format --check src tests templates/agent-langgraph templates/ui/agent-langgraph
• uv run ty check
• node --check ui/app.js
• headless Chrome verification at 1440×900 with a 30-message transcript

Account-routed workspace support

• generated SDK clients forward DATABRICKS_WORKSPACE_ID as X-Databricks-Org-Id for account/vanity-host profiles
• the generated LangGraph model adapter also forwards that routing header to the underlying sync and async OpenAI clients
• MCP transports receive the same routing header without changing integrations/openai

Demo app

• https://mason-pr487-185019-6051921418418893.staging.aws.databricksapps.com

Additional E2E validation · August 28, 2026

• clean editable Mason install and fresh mason init --framework langgraph --enable-chat-app scaffold
• generated project: 30 passed, 1 skipped
• local Apps proxy: session create/list/open, message-only transcript hydration, sync, streaming, background, memory create/list/search, and HITL resume
• stop/start recovery retained tool_step_1 and tool_step_2, then resumed only tool_step_3 and tool_step_4 on a new process
• deployed mason-pr487-185019: deployment SUCCEEDED, app RUNNING, compute ACTIVE