Public workspace document chat fails when workspace is hidden

[paullizer]

Issue

When a user can access a public workspace but has hidden it from the public workspace directory/chat scope, opening that workspace directly and choosing Chat on a document carries the document selection into the chat UI, but the document is not actually searched.

Steps to Reproduce

1. Mark an accessible public workspace as hidden in the public workspace directory.
2. Open that public workspace directly.
3. Choose Chat for a processed document, or select documents and choose the bulk chat action.
4. Ask a question that should be answered from the selected document.

Expected Behavior

The chosen workspace is added to the current user's visible public workspaces without hiding any other visible workspaces. It appears in the chat workspace selector, and the selected document is included in grounded search.

Actual Behavior

The chat page can appear to carry the document selection, but the hidden workspace is absent from the chat selector and the visible-workspace documents endpoint excludes its documents, so the answer is not grounded in the selected file.

Impact

Users receive misleading UI state and answers that silently omit the document they explicitly chose.

Notes

The public workspace document and bulk-chat handoffs navigate with workspace_id, while chat document loading is intentionally filtered by the user's publicDirectorySettings. The repository already has additive public-workspace visibility helpers, so the fix should preserve existing visible workspaces.

[paullizer]

Implemented in the working tree for v0.250.200.

What changed

• The /chats route now detects explicit public document-search handoffs before building the public workspace selector.
• It resolves the caller-supplied workspace ID and revalidates access before additively enabling that workspace in the user's publicDirectorySettings.
• Existing visible workspaces are preserved, and already-visible workspaces avoid a redundant write.
• Added focused regression coverage, fix documentation, and a v0.250.200 release-note entry.

Validation

• python functional_tests/test_public_workspace_hidden_document_chat_visibility.py: 5/5 passed
• python -m py_compile application/single_app/route_frontend_chats.py functional_tests/test_public_workspace_hidden_document_chat_visibility.py: passed
• Route Blueprint policy inventory: 6/6 passed
• Route unauthenticated policy contract: 4/4 passed
• Route policy test coverage: 2/2 passed
• Pylance syntax checks: no errors

[paullizer]

Shipped in v0.250.200 via #1246 and #1248.

Release note entries:

• v0.250.200: Hidden Public Workspace Document Chat Grounding

Full notes: https://microsoft.github.io/simplechat/explanation/release_notes/

Closing as completed. This issue stayed open after the fix merged because pull requests target Development while the repository default branch is main, so GitHub never applied the Fixes # keyword automatically.