fix(mcp): add full column validation to update_chart

update_chart was only running SchemaValidator + Tier-2 compile check,
silently skipping DatasetValidator's column-existence + fuzzy-match
and column-name normalisation layers that generate_chart runs.

A typo like {name: "reveneu"} would save the broken chart and only
surface as a render-time failure in the browser.

Now matches generate_chart pipeline:
- Layer 2: DatasetValidator.validate_against_dataset() — column
  existence check with fuzzy-match "did you mean?" suggestions returned
  to the LLM before any DB write occurs
- Layer 4: DatasetValidator.normalize_column_names() — case
  normalisation so "order_date" resolves to "OrderDate" if that is the
  canonical dataset name

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>