[Release Plan Dashboard] Hide "Link a different SDK PR" guidance for private preview release plans

## Summary

On the release plan dashboard, the release plan detail view renders a standalone "Link a different SDK PR" section at the bottom of every non-finished plan. Private preview release plans do not generate or release SDKs, so this guidance does not apply and is confusing for those plans. It should be suppressed for private preview release plans.

## Current behavior

For a private preview release plan, the detail view still shows:

> 💡 **Link a different SDK PR:** Use the Azure SDK Tools agent in Copilot CLI or VS Code and run:
> `Link SDK pull request to release plan `

This directly contradicts the dashboard's own private preview messaging elsewhere in the same view: "SDKs are not generated or released for private preview release plans." (`public/app.js` ~line 1846).

## Expected behavior

Do not render the "Link a different SDK PR" section for private preview release plans. It should continue to render as-is for Public Preview and GA plans.

## Implementation notes

- File: `tools/release-plan-dashboard/public/app.js`
- The block is rendered around lines 2108-2115, currently gated only on `p.releasePlanId && p.state !== "Finished"`.
- A helper already exists for this check: `isPrivatePreviewPlan(p)` (~line 412), which matches `releasePlanType` containing "private".
- Fix: add `&& !isPrivatePreviewPlan(p)` to the render condition.

## Testing

- Add/extend a Vitest case (see `tests/package-feed.test.js` for existing private preview coverage) to assert the "Link a different SDK PR" block is omitted for a `releasePlanType: "APEX Private Preview"` plan and present for a GA plan.

## Impact / effort

Low effort, low risk. Cosmetic/guidance-only change scoped to one conditional in the frontend, backed by an existing helper.

Start in tools/release-plan-dashboard/public/app.js at the render block around lines 2108-2115, and review the existing isPrivatePreviewPlan helper around line 412. Run or extend tests/package-feed.test.js to verify the guidance is absent for an APEX Private Preview plan and remains present for a GA plan.

Written by the indexing model from the issue text.