fix(chat): route /api/repos/{repo_id}/chat/messages by workspace_sessions

[enyr-srl]

fix(chat): route /api/repos/{repo_id}/chat/messages by workspace_sessions

Summary

In workspace mode, every non-primary repo's chat returns
404 Repository <id> not found even though the same id is listed
in both GET /api/repos and GET /api/workspace.repos[].repo_id.
The chat router was the only /api/repos/{repo_id}/... endpoint
not honouring app.state.workspace_sessions.

The conversation endpoints (GET /chat/conversations, etc.)
already work correctly because they use Depends(get_db_session),
which routes by repo_id automatically. chat_messages couldn't
use that dep — it needs the factory (not a single session) so it
can open multiple sessions over the SSE lifetime — so it was
hand-wiring request.app.state.session_factory and missing the
workspace branch.

Fix

• Extract the factory-resolution logic from get_db_session into
  two reusable helpers in deps.py:
  • resolve_session_factory(app_state, repo_id) — pure
    function, same logic as the existing private helper in
    routers/repos.py.
  • resolve_request_session_factory(request) — request-scoped
    sibling that reads repo_id from path/query params (mirrors
    what get_db_session encodes).
• chat_messages calls resolve_request_session_factory(request)
  instead of request.app.state.session_factory directly.
• routers/repos.py::_resolve_repo_session_factory is now a
  one-line alias around the shared helper, removing the duplicate
  logic.

Tests

New regression test tests/unit/server/test_chat_workspace.py:

• test_chat_messages_resolves_non_primary_repo_in_workspace_mode
  — POST to a non-primary repo_id no longer returns 404.
  Verified to fail on the unpatched source with the exact
  Repository <id> not found message.
• test_chat_messages_still_finds_primary_repo — the
  single-factory fallback still works when workspace_sessions is
  empty (single-repo mode and the primary repo of a workspace).

All 126 existing tests/unit/server/ tests pass with the patch
(verified locally).

Repro (against an unpatched workspace deployment)

curl -X POST \
  http://localhost:7337/api/repos/<non-primary-id>/chat/messages \
  -H "Content-Type: application/json" \
  -d '{"message":"hi"}'
# {"detail":"Repository <non-primary-id> not found"}

The same <non-primary-id> appears in
GET /api/workspace.repos[].repo_id.

Test plan

• New regression test fails on unpatched source with the exact
  user-visible 404 message
• New regression test passes after the fix
• All existing tests/unit/server/ tests still pass
• Manual smoke test against a 2-repo workspace from a fresh
  install (reviewer)

Closes #145.