Add native aggregate public chat to V2

[Paul Lizer (paullizer)]

Summary

• Replace the public directory's classic-chat hand-off with native V2 chat and add persistent All and Visible public-workspace search scopes.
• Keep both scopes public-only and server-resolved across follow-ups, conversation replay, orchestration, delegated-agent calls, and live conversation locks; New chat resets the choice.
• Prevent misleading whole-workspace chips from being silently ignored in aggregate mode; update feature documentation and bump the app version to 0.261.310.

Linked issue

Refs #1722 (row 3 only; the broader issue remains open)

Release Notes & Latest Features

• New Feature
• Bug Fix
• UI Enhancement
• Breaking Change
• Internal only

Is this visible to end users?

• Yes
• No

Is this admin-facing (Admin Settings, governance, deployment, config)?

• Yes
• No

Should this become a Latest Feature card?

• Yes
• No
• Already added

Screenshot needed for the card?

• Yes
• No
• Attached

Version bump

• application/single_app/config.py VERSION third segment bumped, or not needed because this is docs-only
• deployers/version.txt bumped, or not needed because deployers/ was not changed

Testing / validation

• npm --prefix .\application\v2_ui run typecheck - passed.
• npm --prefix .\application\v2_ui run build - passed (existing large-chunk size warning).
• python -m pytest .\ui_tests\test_v2_public_chat_scope.py .\ui_tests\test_v2_chat_context_selection.py .\ui_tests\test_v2_public_directory.py -q --tb=short - 109 passed.
• python -m pytest .\functional_tests\test_v2_public_chat_scope.py -q --tb=short - 27 passed; python -m pytest .\functional_tests\test_orchestration_scope_reference_authorizer.py -q --tb=short - 31 passed, 44 subtests passed; python -m pytest .\functional_tests\test_agent_delegation_runtime.py -q --tb=short - 41 passed.
• python .\functional_tests\test_v2_chat_context_picker.py - 10/10 passed; python .\functional_tests\test_v2_new_chat_scoping.py - 6/6 passed; python .\functional_tests\test_document_search_action_scope_enforcement.py - passed.
• python .\functional_tests\route_tests\test_route_blueprint_policy_inventory.py - 15/15 passed; python .\functional_tests\route_tests\test_route_unauthenticated_policy_contract.py - 10/10 passed; python .\functional_tests\route_tests\test_route_policy_test_coverage.py - 3/3 passed.
• python .\functional_tests\test_docs_app_surface_coverage.py - 7/7 passed; python .\functional_tests\test_docs_site_quality.py - 6/6 passed. Changed-line XSS and broken-access-control scans and git diff --check passed.
• Live Azure and companion "New UI only" end-to-end checks were unavailable. A broader unrelated group-workspace fixture run cannot initialize in this local environment because the installed Flask expects werkzeug.__version__, which the installed Werkzeug does not expose.

Documentation

• Release notes updated, or not needed (release-note update remains pending maintainer decision)
• Feature documentation updated, or not needed
• Fix documentation updated, or not needed (feature implementation; no standalone fix doc needed)

Security checklist

• New Flask routes include @swagger_route(security=get_auth_security()) (no new Flask routes)
• Settings sent to non-admin frontends use sanitize_settings_for_user() (no raw settings sent)
• Browser JavaScript is served from local SimpleChat static assets only; no CDN-hosted JS
• No secrets, keys, connection strings, or local-only artifacts are included