Give the model configuration observation fixture its placement rules

[RyanLee-Dev]

backend (make check-core) has failed on every main run since #342 (ffe6734e), for example run 36815507655, which blocks every PR's required check:

--- FAIL: TestObservationExcludesOtherSourcesAndHistoricalSessions
    observation_test.go:95: session create session: hosted Session creation requires placement rules
FAIL	.../persistence/postgres/modelconfigurationpg

What changes

#342 made Store.CreateSession require placement rules for an openai_hosted Session; cmd/server supplies them through SetPlacement. The modelconfigurationpg observation fixture builds its own store.Store and was not updated, and its non-deployment cases create hosted Sessions. The fixture now calls SetPlacement with the rules cmd/server and the store fixtures build: placement.NewRules(providers.Builtin(), ""). With no deployment configured, admission passes as it does in production. Test-only change.

Checks

• Reproduced the failure on origin/main against a local PostgreSQL with OAC_TEST_DATABASE_URL; with this change the whole modelconfigurationpg package passes.
• go vet on the package.
• The two Python steps of check-core that have not run in CI since the break (official_diagnostics_test.py, managed_init_test.py) pass locally.
• The rest of the Go suite already passed in the failing CI runs; this PR's CI is the full check.

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