feat(appkit): introduce modular SDK wrapper (PoC)

[MarioCadenas]

Wrap @databricks/sdk-experimental behind an AppKit-internal facade (packages/appkit/src/workspace-client/) and wire the modular Databricks SDK packages where viable.

Migrated service clients:

• wrapper.files → @databricks/sdk-files (camelCase fields, mapping adapter at the connector boundary for AppKit's public types)
• wrapper.warehouses → @databricks/sdk-warehouses
• wrapper.vectorSearch → @databricks/sdk-vectorsearch
• wrapper.http → @databricks/sdk-core/http + @databricks/sdk-auth (replaces apiClient.request for SCIM probe, serving SSE, internal telemetry; native AbortSignal — no Context bridge)

Delegated to legacy SDK via wrapper.toLegacyWorkspaceClient() with TODO(prod) markers:

• wrapper.genie / wrapper.jobs (modular packages exist but the connectors need a non-trivial rewrite — method renames + waiter idiom + camelCase fields)
• wrapper.statementExecution / wrapper.servingEndpoints / wrapper.currentUser (no modular package yet)
• .toLegacyWorkspaceClient() bridge for @databricks/lakebase

Unified error handling — isApiError(e) / getApiErrorStatusCode(e) helpers work across both old-SDK and modular ApiError shapes (modular SDK throws @databricks/sdk-core/apierror; old SDK throws @databricks/sdk-experimental ApiError; instanceof can't unify them).

Upstream packaging-bug workarounds in patches/@databricks__sdk-*.patch (wired via pnpm-workspace.yaml patchedDependencies):

• Add .js extensions to extensionless ESM imports across dist/
• Add "default" condition to package.json exports (CJS-resolver support, needed by dev-playground's tsx)
• Fix parseResponse to handle HTTP 204 empty bodies (was throwing "Error: [object Object]" on every void-response endpoint)

Incidental AppKit bug fix surfaced during testing — skipBodyParsing path matching used exact-string Set lookup against route templates (/api/files/:volumeKey/upload), which never matched concrete URLs (/api/files/files/upload), so every file upload bombed in body-parser. Replaced with compiled-regex matching.

Tests: 2591 passed | 51 skipped. Skipped suite is
connectors/files/tests/client.test.ts — heavy legacy-shape mocks need a rewrite against modular FilesClient methods (TODO(prod)).

Verified end-to-end against staging:

• dev-playground boots, 42 routes, agents load
• listDirectoryContentsIter, downloadFile, getFileMetadata, createDirectory, deleteDirectory all hit Databricks successfully
• listWarehouses returns real warehouse list
• Modular ApiError preserved through cache layer (no more 500 masking of real 404s)