[Feature] No local/offline validator for item definitions before updateDefinition

@AmosHersch is already working on this.

Since Aug 18, 2026.

**Describe the bug**
This is a feature request. There is no `datafactory_validate-pipeline` / `core_validate-item-definition` style tool to check an item definition payload against the Fabric schema before calling `updateDefinition`. Every schema mistake (wrong activity type, missing required field, wrong enum value, etc. -- see linked issues) required a live round trip to the service (and in one case, only surfaced in the portal UI, since the API itself reported success) to discover the problem.

**Expected behavior**
A tool that validates a definition payload (e.g. a `dataPipeline` item's `pipeline-content.json`) against the documented schema locally/offline, returning specific field-level errors, before it's ever sent to `updateDefinition`.

**Actual behavior**
Multiple mistakes that a schema validator would have caught immediately instead required live API round trips (and once, opening the portal UI) to discover:
- An activity type (`RunNotebook`) that isn't valid for the item type, but is accepted and stored anyway, only failing silently in the UI renderer
- A required-but-undocumented field (`externalReferences.connection` for `TridentNotebook` activities)
- An undocumented enum value (`payloadType: "InlineBase64"`) for definition parts

**Reproduction Steps**
1. Author any Fabric item definition (pipeline, notebook, etc.) with a subtle schema mistake
2. Call `updateDefinition` -- it may succeed even though the item is unusable at runtime
3. Note there is no tool to catch this before the round trip

**Environment**
- Fabric MCP Server
- Windows 11