[Gavin Barron (gavinbarron)]

Description

Replace the legacy query-string instructions in Testing of Sample Queries with the Graph Explorer Custom samples profile-menu workflow. Update the contributing guide's references to use the same canonical instructions.

Document Microsoft-account eligibility, organization and branch selection, the optional trusted HTTPS service URL, apply/reset behavior, loading errors, dismissal, session-only settings, source scope, and request-data disclosure.

Consumer impact: preview links using devx-api, org, and branchName no longer select custom sources. The replacement workflow requires an authenticated account whose username ends exactly in @microsoft.com; this UI guard is not server-side authorization.

This PR targets master and is intentionally a draft. It depends on the matching Graph Explorer implementation and rollout. Keep it in draft until the new workflow is available on the linked Graph Explorer site.

No sample catalogs, permissions, translations, application code, or publication pipelines are changed.

Testing Done

• git diff --check: passed.
• Manually reviewed the README and contributing-guide changes; the existing README.md#testing-of-sample-queries anchor is preserved.
• npm run test -- --no-watch: not run (N/A for this documentation-only change). The repository's Jest and schema checks validate JSON content, which is unchanged; no Markdown-specific test command is configured.

Checklist

• The change is scoped to the intended content or configuration.
• Applicable tests were added or updated and pass. (N/A: documentation-only change; content tests were not run.)
• Applicable documentation was updated.
• Breaking changes and consumer impact are described.
• No secrets or sensitive data are included.