Turn failures don't name the provider and model that failed, and provider origin is not exposed to clients #4328

Summary

A request can be routed to a provider the user didn't pick and can't delete in the app, e.g. a custom provider defined in a models.json or an extension (sisyphuslabs/omo-desktop-app#4711). The failure says "The provider rejected the credentials. Action: re-authenticate the provider." and names neither provider nor model (packages/coding-agent/src/core/provider-failure.ts:13-22,76-78). The structured failure carries no model and describes only the last fallback rung. Clients get no file path or owning extension for a provider (modes/rpc/rpc-session-state-types.ts:22-49, rpc-read-response-types.ts:47-51, core/model-runtime.ts:1467-1484), so the app can't say where an entry comes from.

Expected

• The turn failure (agent_end.failure and RPC payloads) carries the provider id and model actually used for the failing request, plus the provider the user selected when they differ.
• Provider listings expose the origin (models.json path / env var name / extension id) as machine-readable fields.
• TUI text names provider and model.

Related

sisyphuslabs/omo-desktop-app#4711, #4084, #2491.

Part of sisyphuslabs/omo-desktop-app#4713 (error-guidance audit; bases desktop 5d85e0d8bb, oh-my-openagent 7c0fed935c, senpi a854e7a, omo-service b709624). Tests pin only machine-consumed values (enum/code -> state mapping), never prose.