create_tunnel does not validate that source/target rooms exist

[yonefive71]

Summary

palace_graph.create_tunnel validates that wing/room names are non-empty strings (via _require_name) but never confirms that the named rooms actually contain any drawers. Pointing an explicit tunnel at a phantom room — agent fabricates a room name it expects to exist, typo in a slug, scaffolding before mining — silently succeeds and persists.

Combined with #1467 (tunnel-file ignores palace_path), this lets an isolated agent both write and read its own bogus tunnel and self-confirm "tunnel created" in completion reports.

Reproduction

import mempalace.mcp_server as ms
ms.tool_add_drawer("wing_real", "real_room", "content")
# target_room is wrong, but create_tunnel doesn't care:
result = ms.tool_create_tunnel("wing_real", "real_room", "wing_real", "phantom_room")
print(result)   # success, tunnel persisted, pointing at a room with zero drawers

Fix

PR #TBD. For kind == "explicit", create_tunnel now queries the chroma collection with col.get(where={"$and": [{"wing": W}, {"room": R}]}, limit=1, include=[]) for both endpoints. Zero rows → raises ValueError naming the offending endpoint.

Three carve-outs preserved:

1. kind != "explicit" — topic tunnels use synthetic topic:<name> room IDs that don't appear in chroma. Skip.
2. _get_collection returns None (palace not yet created, tests, transient outage) — skip rather than fail-closed, matches palace_graph tolerance elsewhere.
3. Query raises — log and treat as "can't verify, allow" so a flaky index doesn't block legitimate writes.

Behaviour change: existing callers that previously created tunnels pointing at empty rooms as scaffolding will now raise. File the drawer first, then create the tunnel — the order the docs have always recommended.

[igorls]

Fixed by PR #1469 (merged as ae8074e). create_tunnel now validates both endpoints via _check_room_exists when kind=="explicit", raising ValueError naming the offending wing/room. Topic/entity tunnels skip validation since they can legitimately precede their rooms. Closing.