fix(mcp): surface backend errors in Agent UI MCP callers + stop session nav oscillation

[github-actions]

The #1750 review landed several findings after merge, so these regressions are currently on main. Two of them violate the "No Silent Fallbacks" rule: after #1750 switched MCP boundary errors to the {"status":"error","detail":...} envelope, get_messages and index_document still keyed on the old {"error"} shape — so a backend failure (e.g. a 404 for a missing session) silently fell through to an empty {"messages":[],"total":0} payload, and a failed session-document attach was reported as a successful link. Both now detect the envelope and fail loudly. The third is a 🔴 UI regression: the App.tsx URL-hash nav guard re-ran on every currentSessionId change, read a stale 7-char short hash (which never equals the full UUID), and scheduled a ~500ms bounce that could ping-pong sessions — breaking the very MCP→UI activation flow #1750 added. The guard now runs once on mount.

Also folds in the review's minor items: delete_session and the screenshot tool now use the same structured envelope, and the unused emit_set_active_session shim is removed. The tautological App.test.tsx (literal-vs-literal assertions that never rendered App) is replaced with a real render-based regression test for the nav guard.

Closes #1755

Test plan

• python -m pytest tests/unit/test_agent_ui_mcp.py — 13 passed (incl. new get_messages silent-fallback and index_document false-success regression tests)
• python -m pytest tests/unit/ — 5231 passed (1 pre-existing, unrelated test_memory_router FAISS failure also present on main)
• python util/lint.py --flake8 / --black / --isort clean
• cd src/gaia/apps/webui && npm run test — runs the new App.test.tsx nav-guard test (vitest; node_modules not available in this job)
• Live A→B session switch: open the Agent UI, activate session A via the MCP bridge, switch to B in the sidebar, confirm it stays on B (no bounce back to A) and B's messages aren't wiped

⚠️ Needs manual validation — the automated checks here confirm no Python regression but can't exercise the frontend. A maintainer should run npm run test in src/gaia/apps/webui and do the live A→B session-switch check above before merging.