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.test.ts

@@ -0,0 +1,77 @@
+import process from "node:process"
+import { afterEach, describe, expect, it } from "bun:test"
+
+import { resolveActualContextLimit } from "./context-limit-resolver"
+
+const ANTHROPIC_CONTEXT_ENV_KEY = "ANTHROPIC_1M_CONTEXT"
+const VERTEX_CONTEXT_ENV_KEY = "VERTEX_ANTHROPIC_1M_CONTEXT"
+
+const originalAnthropicContextEnv = process.env[ANTHROPIC_CONTEXT_ENV_KEY]
+const originalVertexContextEnv = process.env[VERTEX_CONTEXT_ENV_KEY]
+
+function resetContextLimitEnv(): void {
+  if (originalAnthropicContextEnv === undefined) {
+    delete process.env[ANTHROPIC_CONTEXT_ENV_KEY]
+  } else {
+    process.env[ANTHROPIC_CONTEXT_ENV_KEY] = originalAnthropicContextEnv
+  }
+
+  if (originalVertexContextEnv === undefined) {
+    delete process.env[VERTEX_CONTEXT_ENV_KEY]
+  } else {
+    process.env[VERTEX_CONTEXT_ENV_KEY] = originalVertexContextEnv
+  }
+}
+
+describe("resolveActualContextLimit", () => {
+  afterEach(() => {
+    resetContextLimitEnv()
+  })
+
+  it("returns the default Anthropic limit when 1M mode is disabled despite a cached limit", () => {
+    // given
+    delete process.env[ANTHROPIC_CONTEXT_ENV_KEY]
+    delete process.env[VERTEX_CONTEXT_ENV_KEY]
+    const modelContextLimitsCache = new Map<string, number>()
+    modelContextLimitsCache.set("anthropic/claude-sonnet-4-5", 123456)
+
+    // when
+    const actualLimit = resolveActualContextLimit("anthropic", "claude-sonnet-4-5", {
+      anthropicContext1MEnabled: false,
+      modelContextLimitsCache,
+    })
+
+    // then
+    expect(actualLimit).toBe(200000)
+  })
+
+  it("treats Anthropics aliases as Anthropic providers", () => {
+    // given
+    delete process.env[ANTHROPIC_CONTEXT_ENV_KEY]
+    delete process.env[VERTEX_CONTEXT_ENV_KEY]
+
+    // when
+    const actualLimit = resolveActualContextLimit(
+      "aws-bedrock-anthropic",
+      "claude-sonnet-4-5",
+      { anthropicContext1MEnabled: false },
+    )
+
+    // then
+    expect(actualLimit).toBe(200000)
+  })
+
+  it("returns null for non-Anthropic providers without a cached limit", () => {
+    // given
+    delete process.env[ANTHROPIC_CONTEXT_ENV_KEY]
+    delete process.env[VERTEX_CONTEXT_ENV_KEY]
+
+    // when
+    const actualLimit = resolveActualContextLimit("openai", "gpt-5", {
+      anthropicContext1MEnabled: false,
+    })
+
+    // then
+    expect(actualLimit).toBeNull()
+  })
+})