fix(appkit): obo logic and api usage

[MarioCadenas]

Summary

Introduces a clean asUser(req) API for executing Databricks operations with user credentials. Replaces the middleware-based context management with a declarative pattern.

What Changed

New: plugin.asUser(req).method()` pattern for user-scoped execution.

New: ServiceContext singleton initialized at app startup.

New: ExecutionContext using AsyncLocalStorage for transparent context propagation

Removed: databricksClientMiddleware (no longer needed)

Removed: requiresDatabricksClient plugin flag

Architecture

┌─────────────────────────────────────────────────────────────┐
│                      App Startup                            │
│  • createApp() → ServiceContext.initialize()                  │
│  • Creates service principal WorkspaceClient                │
│  • Fetches warehouseId, workspaceId                         │
└─────────────────────────────────────────────────────────────┘
                              │
                              ▼
┌─────────────────────────────────────────────────────────────┐
│                     Request Handling                        │
├─────────────────────────────────────────────────────────────┤
│                                                             │
│  Service Principal Path       User Context Path             │
│  ───────────────────────      ─────────────────────────     │
│                                                             │
│  analytics.query(...)         analytics.asUser(req)         │
│         │                            │                      │
│         ▼                            ▼                      │
│  ServiceContext.get()         Creates IUserContext from     │
│         │                     x-forwarded-access-token      │
│         ▼                            │                      │
│  Uses service principal       Returns Proxy that wraps      │
│  WorkspaceClient              methods in AsyncLocalStorage  │
│                                      │                      │
│                                      ▼                      │
│                               .query(...) executes with     │
│                               user's WorkspaceClient        │
│                                                             │
└─────────────────────────────────────────────────────────────┘

Usage

// Service principal (default)
const data = await analytics.query("SELECT ...");

// User context (on behalf of user)
const userData = await analytics.asUser(req).query("SELECT ...");

Frontend

Charts/tables now support asUser prop:

<LineChart queryKey="spend" parameters={params} asUser />

[pkosiec]

LGTM - please see those small comments before merge. Thanks!