Remove toolbox preview header from the Mastering Toolbox notebook

[lindazqli]

What

The mastering-foundry-toolbox.ipynb notebook applied a Foundry-Features: Toolboxes=V1Preview header (via a TOOLBOX_HEADERS constant) to every toolbox MCP call, the management REST API, and skill-resource reads. That header is not required for any of those paths. This PR removes it everywhere and corrects the surrounding prose.

Why

• Tool consumption (MAF, LangGraph, Copilot SDK, raw MCP tools/list / call_tool) needs only a bearer token scoped to https://ai.azure.com/.default - no preview header.
• Toolbox management REST (POST .../versions, PATCH .../{name}) needs only the bearer token.
• Consuming skills as MCP resources (10b, resources/list / resources/read) is standard MCP against the same endpoint - no preview header.
• The only flow that needs a preview feature flag is skills CRUD (project.beta.skills.* create / update-default), and the beta.skills SDK sends it automatically - so it is never set by hand in the notebook.

Changes

• Removed the TOOLBOX_HEADERS definition and every **TOOLBOX_HEADERS spread (5 REST, verify cells, 10b skills, MAF, LangGraph, Copilot SDK).
• Updated intro, 5, 9, and 4l prose accordingly.

No tool behavior changes - the removed headers were unnecessary. Notebook JSON validated; all 59 cells intact.

[nicholasdbrady]

Cookbook review: Publish after fixes

The general Toolboxes=V1Preview cleanup is consistent, but the Skills path still needs one correction before merge.

• Required: Section 4l · Skills now says Skills CRUD is the only flow that needs a preview feature flag. Current Microsoft Learn guidance also requires Foundry-Features: Skills=V1Preview when a toolbox version includes a skill reference. This notebook pins azure-ai-projects 2.3.x and calls project.toolboxes.create_version(..., skills=...); that stable operation does not inject the Skills header. With SKILL_NAME set, the version-creation cells can therefore fail. Conditionally pass headers={"Foundry-Features": "Skills=V1Preview"} whenever skills is non-empty (including the normal, search-first, and RAI-guarded version creation paths), and update the prose so it does not claim only Skills CRUD is preview-gated. The optional REST example currently has no skills payload, so it does not need the header unless that payload is extended.

After the fix, exercise the notebook path with SKILL_NAME set to confirm skill attachment succeeds.

What works: The obsolete toolbox header constant and its MCP/management call sites were removed consistently; notebook cell IDs, metadata, outputs, and execution counts remain unchanged, and the PR checks pass.