Restore per-turn model fallback removed in #323 (ensureResolvableModel / ModelFallbackSwitched)

[elkaix]

#299 added a turn-start check in agent-core-v2 profileService.resolveModelContext. When a session's bound model no longer resolves, it switched to the best ready model and emitted ModelFallbackSwitched plus the model_fallback_triggered telemetry event. #323 (upstream reconcile) removed the check, the event, and its consumers.

Effect today: a session bound to a deleted or unconfigured model fails its turn instead of falling back. The global default-model fallback (settleDefaultModel) is being restored separately in the legacy-kosong removal PR.

Source: git show 62ae31bfd, 96d1a0c7a^:packages/agent-core-v2/src/agent/profile/profileService.ts:411-460.

[elkaix]

Scope addition from the legacy-kosong removal. The global default-model fallback (settleDefaultModel + defaultModelPolicy) also exists only in the deleted legacy copy. It replaced a default that no longer resolves with the last-used model, else the best-ranked ready model, and published a ConfigWarning. I did not restore it, because #323 rebuilt agent-gateway/test/modelCatalogProviderWrite.test.ts around the opposite behavior (default_model is never rewritten). Restoring it is a product decision. The source and tests are at 96d1a0c7a^:packages/agent-core-v2/src/kosong/model/{modelService,defaultModelPolicy}.ts and test/kosong/model/modelService.test.ts.

[elkaix]

Turn-start fallback is restored in #364. The global default-model fallback (settleDefaultModel) is not restored, because #323 made default_model never be rewritten. Bringing it back is a product decision. Open a new issue if it is wanted.