feat(config): support object-style fallback_models with per-model settings

Add support for object-style entries in fallback_models arrays, enabling
per-model configuration of variant, reasoningEffort, temperature, top_p,
maxTokens, and thinking settings.

- Zod schema for FallbackModelObject with full validation
- normalizeFallbackModels() and flattenToFallbackModelStrings() utilities
- Provider-agnostic model resolution pipeline with fallback chain
- Session prompt params state management
- Fallback chain construction with prefix-match lookup
- Integration across delegate-task, background-agent, and plugin layers

src/hooks/runtime-fallback/fallback-models.ts

@@ -1,10 +1,16 @@
 import type { OhMyOpenCodeConfig } from "../../config"
+import type { FallbackModelObject } from "../../config/schema/fallback-models"
 import { agentPattern } from "./agent-resolver"
 import { HOOK_NAME } from "./constants"
 import { log } from "../../shared/logger"
 import { SessionCategoryRegistry } from "../../shared/session-category-registry"
-import { normalizeFallbackModels } from "../../shared/model-resolver"
+import { normalizeFallbackModels, flattenToFallbackModelStrings } from "../../shared/model-resolver"
 
+/**
+ * Returns fallback model strings for the runtime-fallback system.
+ * Object entries are flattened to "provider/model(variant)" strings so the
+ * string-based fallback state machine can work with them unchanged.
+ */
 export function getFallbackModelsForSession(
   sessionID: string,
   agent: string | undefined,
@@ -12,22 +18,45 @@ export function getFallbackModelsForSession(
 ): string[] {
   if (!pluginConfig) return []
 
+  const raw = getRawFallbackModelsForSession(sessionID, agent, pluginConfig)
+  return flattenToFallbackModelStrings(raw) ?? []
+}
+
+/**
+ * Returns the raw fallback model entries (strings and objects) for a session.
+ * Use this when per-model settings (temperature, reasoningEffort, etc.) must be
+ * preserved — e.g. before passing to buildFallbackChainFromModels.
+ */
+export function getRawFallbackModels(
+  sessionID: string,
+  agent: string | undefined,
+  pluginConfig: OhMyOpenCodeConfig | undefined,
+): (string | FallbackModelObject)[] | undefined {
+  if (!pluginConfig) return undefined
+  return getRawFallbackModelsForSession(sessionID, agent, pluginConfig)
+}
+
+function getRawFallbackModelsForSession(
+  sessionID: string,
+  agent: string | undefined,
+  pluginConfig: OhMyOpenCodeConfig,
+): (string | FallbackModelObject)[] | undefined {
   const sessionCategory = SessionCategoryRegistry.get(sessionID)
   if (sessionCategory && pluginConfig.categories?.[sessionCategory]) {
     const categoryConfig = pluginConfig.categories[sessionCategory]
     if (categoryConfig?.fallback_models) {
-      return normalizeFallbackModels(categoryConfig.fallback_models) ?? []
+      return normalizeFallbackModels(categoryConfig.fallback_models)
     }
   }
 
-  const tryGetFallbackFromAgent = (agentName: string): string[] | undefined => {
+  const tryGetFallbackFromAgent = (agentName: string): (string | FallbackModelObject)[] | undefined => {
     const agentConfig = pluginConfig.agents?.[agentName as keyof typeof pluginConfig.agents]
     if (!agentConfig) return undefined
-    
+
     if (agentConfig?.fallback_models) {
       return normalizeFallbackModels(agentConfig.fallback_models)
     }
-    
+
     const agentCategory = agentConfig?.category
     if (agentCategory && pluginConfig.categories?.[agentCategory]) {
       const categoryConfig = pluginConfig.categories[agentCategory]
@@ -35,7 +64,7 @@ export function getFallbackModelsForSession(
         return normalizeFallbackModels(categoryConfig.fallback_models)
       }
     }
-    
+
     return undefined
   }
 
@@ -53,5 +82,5 @@ export function getFallbackModelsForSession(
 
   log(`[${HOOK_NAME}] No category/agent fallback models resolved for session`, { sessionID, agent })
 
-  return []
+  return undefined
 }