feat: add React external sharing sensitive review

[vystartasv]

Description

Adds a bounded, read-only React External Sharing and Sensitive Content Review web part for SPFx 1.23.2.

What changed

• Reads configured SharePoint list/library rows with SharePoint REST GET only.
• Surfaces heuristic indicators for external/guest access, broad sharing links, sensitive-content labels/tags, unique permissions, missing governance metadata, and stale review dates.
• Enforces four sources, 50 rows per page, five pages, and 200 rows per source.
• Validates server-relative paths, same-origin HTTPS/root-relative item links, and OData next links.
• Handles malformed rows, optional fields, empty/partial results, access denial, throttling, retry information, loading, and retry states.
• Provides severity/source filters, evidence text, semantic table markup, keyboard-accessible controls, and responsive styling.
• Includes deterministic tests and a static read-only verifier.
• Does not change permissions, sharing, labels, compliance tags, content, or metadata; no Graph permissions or secrets.

Validation

• npm ci
• npm test — 7 tests passed.
• npm run build — SPFx bundle passed.
• npm run package — production .sppkg passed.
• npm run verify — read-only verifier passed after generated-artifact cleanup handling was corrected.
• Gallery metadata, configuration, scope, whitespace, and generated-artifact checks passed.

Tenant validation requested

Please validate representative SharePoint Online lists/libraries and real tenant data, including external/guest indicators, sharing-link values, sensitivity/compliance fields, unique permissions, missing optional fields, stale dates, managed paths, malformed rows, paging bounds, empty results, partial failures, permission denial, throttling, keyboard access, and responsive layout. Real tenant screenshots are welcome; none are fabricated locally.

[vystartasv]

Could someone with a SharePoint Online tenant please validate PR #6490 and add real screenshots if possible?

Please test representative document-library/list endpoints and fields for external or guest access, broad sharing-link indicators, sensitivity/compliance metadata, unique permissions, missing optional fields, stale dates, managed paths, malformed rows, paging bounds, empty results, partial failures, permission denial, throttling, keyboard access, and responsive layout.

The sample is a bounded heuristic review aid using SharePoint REST GET requests only. It does not change sharing, permissions, labels, compliance tags, content, or metadata. It does not assert authoritative Purview or tenant security state. No screenshots are fabricated locally.

[github-actions]

Sample PR validation for #6490

⚠️ Validation status: warnings

We automatically validate all pull requests against our contribution guidance to ensure that all samples provide a consistent experience to our community.

In order to merge this PR in a timely manner, the following criteria must be met:

Validation	Status
Pull request affects only one folder	✅ Succeeded
Sample folder name follows naming convention	✅ Succeeded
README.md contains visitor stat image	⚠️ Warning
Sample requires a .nvmrc file	⚠️ Warning
Sample requires a README.md	✅ Succeeded
Sample requires a screenshot .png in assets folder	⚠️ Warning
No .sppkg file	✅ Succeeded
No node_modules folder	✅ Succeeded
No lib folder	✅ Succeeded
No upgrade reports	✅ Succeeded

@vystartasv please address the above issues and push new changes to this branch.
For more information, see the contribution guidance.

[github-actions]

Sample PR validation for #6490

⚠️ Validation status: warnings

We automatically validate all pull requests against our contribution guidance to ensure that all samples provide a consistent experience to our community.

In order to merge this PR in a timely manner, the following criteria must be met:

Validation	Status
Pull request affects only one folder	✅ Succeeded
Sample folder name follows naming convention	✅ Succeeded
README.md contains visitor stat image	✅ Succeeded
Sample requires a .nvmrc file	✅ Succeeded
Sample requires a README.md	✅ Succeeded
Sample requires a screenshot .png in assets folder	⚠️ Warning
No .sppkg file	✅ Succeeded
No node_modules folder	✅ Succeeded
No lib folder	✅ Succeeded
No upgrade reports	✅ Succeeded

@vystartasv please address the above issues and push new changes to this branch.
For more information, see the contribution guidance.