Removes module-level core.database stubbing from the compare endpoint owner-scope regression test and patches ModelEndpoint per test with monkeypatch. Restores one focused part of the Python CI baseline tracked in #2580.
POST /api/compare/start (a normal-user route — no admin gate) creates two
caller-owned [CMP] sessions from caller-supplied endpoint URLs (endpoint_a /
endpoint_b), then copies a ModelEndpoint's *decrypted* api_key into each
session's headers by matching on URL:
ep = db.query(ModelEndpoint).filter(ModelEndpoint.base_url == base).first()
The match was not owner-scoped. ModelEndpoint is per-user (core/database.py:
non-null owner = private, "the model picker only shows the endpoint to that
user"). So a user could pass another user's endpoint base_url, have that owner's
api_key copied into a [CMP] session they own, then drive /api/chat_stream on that
session — spending the victim's API key / quota and reaching whatever base_url
they configured. Same multi-tenant owner-scoping class already fixed for
companion/models, /api/v1/chat (#870, #1045), session create/switch-model
(#1093), and /api/research/start (#1099).
Extract `_owned_endpoint_by_url(db, base_url, owner)` which scopes the match via
the shared owner_filter helper (own rows + legacy null-owner shared rows),
mirroring session_routes._owned_endpoint. A scoped miss copies no key (the
comparison session simply carries no borrowed credential). A null/empty owner
stays a no-op (single-user / legacy mode).
Add regression tests pinning the scoped match (cross-owner rejected, own-row
allowed, legacy shared-row allowed, no-match None, null-owner no-op).
