fix(shared): extract shared context limit resolver to eliminate monitor/truncator drift

- New context-limit-resolver.ts with resolveActualContextLimit() shared helper
- Anthropic provider detection now uses .includes('anthropic') instead of hard-coded IDs
- Both context-window-monitor and dynamic-truncator use the shared resolver
- Added missing test cases: Anthropic+1M disabled+cached limit, non-Anthropic without cache

src/shared/context-limit-resolver.ts

@@ -0,0 +1,32 @@
+import process from "node:process"
+
+const DEFAULT_ANTHROPIC_ACTUAL_LIMIT = 200_000
+
+export type ContextLimitModelCacheState = {
+  anthropicContext1MEnabled: boolean
+  modelContextLimitsCache?: Map<string, number>
+}
+
+function isAnthropicProvider(providerID: string): boolean {
+  return providerID.toLowerCase().includes("anthropic")
+}
+
+function getAnthropicActualLimit(modelCacheState?: ContextLimitModelCacheState): number {
+  return (modelCacheState?.anthropicContext1MEnabled ?? false) ||
+    process.env.ANTHROPIC_1M_CONTEXT === "true" ||
+    process.env.VERTEX_ANTHROPIC_1M_CONTEXT === "true"
+    ? 1_000_000
+    : DEFAULT_ANTHROPIC_ACTUAL_LIMIT
+}
+
+export function resolveActualContextLimit(
+  providerID: string,
+  modelID: string,
+  modelCacheState?: ContextLimitModelCacheState,
+): number | null {
+  if (isAnthropicProvider(providerID)) {
+    return getAnthropicActualLimit(modelCacheState)
+  }
+
+  return modelCacheState?.modelContextLimitsCache?.get(`${providerID}/${modelID}`) ?? null
+}