test: align backend and frontend suites with 3-stage numbers pipeline

[guigerdts]

Summary

Updates backend + frontend suites to the 3-stage numbers pipeline contract introduced in #74.

• Rewrites pipeline_service tests: 3-stage report order, removes meta-stage mocks, new healing matrix.
• Deletes test_pipeline_context.py (bt-before-rank) and test_pipeline_autotrain.py (ml/dl auto-train) — both N/A after stage reduction.
• Adds a test that gen succeeds WITHOUT an active MetaSelection (seed fallback).
• frontend/src/types/pipeline.ts: PipelineStageName -> 3 values; MisNumeros.test.tsx updated.

Depends on

#74 (core refactor). Merge #74 first; this PR then targets main.

Verification

Backend (pytest), frontend (npm test), tsc -b, and eslint all pass with both parts landed.