feat(python-notebook-migration): add notebook migration orchestration service

[zyratlo]

What changes were proposed in this PR?

Introduces NotebookMigrationService, the frontend orchestration service that sits between the migration-tool UI and the lower layers: the LLM client (migration-tool-llm-client) and the backend notebook-migration microservice (migration-tool-backend-notebook-migration-service).

notebook-migration.service.ts

• getAvailableModels() — GET /api/models against the existing LiteLLM proxy, returns the model dropdown options.
• sendToAIGenerateWorkflow(notebook, modelType, apiKey) — drives the full NotebookMigrationLLM lifecycle (initialize → verify connection → convert → close in finally) and returns { workflowContent, mappingContent }. Surfaces API-key
  validation failures by throwing.
• sendNotebookToJupyter(notebookData) — POST /api/notebook-migration/set-notebook; surfaces a NotificationService toast on success and failure; returns 1 / 0.
• getJupyterURL(), getJupyterIframeURL() — calls the matching microservice endpoints to retrieve URLs to embed.
• storeNotebookAndMapping(wid, vid, mappingContent, notebookContent) — POST /api/notebook-migration/store-notebook-and-mapping; returns the HttpClient observable directly so callers can compose with switchMap.
• Mapping cache — small in-memory dictionary { [key: string]: MappingContent } keyed by mapping_wid_<workflowId>, with hasMapping, getMapping, setMapping, deleteMapping.

notebook-migration.service.spec.ts

• getAvailableModels: maps the LiteLLM data[].id array correctly; falls back to an empty array on HTTP error.
• sendNotebookToJupyter: success → returns 1; error → returns 0 and toasts.
• getJupyterURL / getJupyterIframeURL: success → returns the URL; non-OK response or thrown error → returns null.
• Mapping cache: setMapping then getMapping round-trips; deleteMapping removes the entry.
• storeNotebookAndMapping: makes the expected POST to the persistence endpoint.

Any related issues, documentation, discussions?

Closes #5261
Parent issue #4301

How was this PR tested?

The new notebook-migration.service.spec.ts adds HttpClientTestingModule-driven test cases

Was this PR authored or co-authored using generative AI tooling?

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