test(background-agent): add unit tests for extracted modules

Add 104 new tests across 4 test files:
- error-classifier.test.ts (80 tests): isRecord, isAbortedSessionError, getErrorText, extractErrorName, extractErrorMessage, getSessionErrorMessage
- fallback-retry-handler.test.ts (19 tests): retry logic, fallback chain, concurrency release, session abort, queue management
- process-cleanup.test.ts (7 tests): signal registration, multi-manager shutdown, cleanup on unregister
- compaction-aware-message-resolver.test.ts (13 tests): compaction agent detection, message resolution with temp dirs (pre-existing, verified)

Total background-agent tests: 161 -> 265 (104 new, 0 regressions)

src/features/background-agent/compaction-aware-message-resolver.test.ts

@@ -0,0 +1,190 @@
+import { describe, test, expect, beforeEach, afterEach } from "bun:test"
+import { mkdtempSync, writeFileSync, rmSync } from "node:fs"
+import { join } from "node:path"
+import { tmpdir } from "node:os"
+import { isCompactionAgent, findNearestMessageExcludingCompaction } from "./compaction-aware-message-resolver"
+
+describe("isCompactionAgent", () => {
+  describe("#given agent name variations", () => {
+    test("returns true for 'compaction'", () => {
+      // when
+      const result = isCompactionAgent("compaction")
+
+      // then
+      expect(result).toBe(true)
+    })
+
+    test("returns true for 'Compaction' (case insensitive)", () => {
+      // when
+      const result = isCompactionAgent("Compaction")
+
+      // then
+      expect(result).toBe(true)
+    })
+
+    test("returns true for ' compaction ' (with whitespace)", () => {
+      // when
+      const result = isCompactionAgent(" compaction ")
+
+      // then
+      expect(result).toBe(true)
+    })
+
+    test("returns false for undefined", () => {
+      // when
+      const result = isCompactionAgent(undefined)
+
+      // then
+      expect(result).toBe(false)
+    })
+
+    test("returns false for null", () => {
+      // when
+      const result = isCompactionAgent(null as unknown as string)
+
+      // then
+      expect(result).toBe(false)
+    })
+
+    test("returns false for non-compaction agent like 'sisyphus'", () => {
+      // when
+      const result = isCompactionAgent("sisyphus")
+
+      // then
+      expect(result).toBe(false)
+    })
+  })
+})
+
+describe("findNearestMessageExcludingCompaction", () => {
+  let tempDir: string
+
+  beforeEach(() => {
+    tempDir = mkdtempSync(join(tmpdir(), "compaction-test-"))
+  })
+
+  afterEach(() => {
+    rmSync(tempDir, { force: true, recursive: true })
+  })
+
+  describe("#given directory with messages", () => {
+    test("finds message with full agent and model", () => {
+      // given
+      const message = {
+        agent: "sisyphus",
+        model: { providerID: "anthropic", modelID: "claude-opus-4-6" },
+      }
+      writeFileSync(join(tempDir, "001.json"), JSON.stringify(message))
+
+      // when
+      const result = findNearestMessageExcludingCompaction(tempDir)
+
+      // then
+      expect(result).not.toBeNull()
+      expect(result?.agent).toBe("sisyphus")
+      expect(result?.model?.providerID).toBe("anthropic")
+      expect(result?.model?.modelID).toBe("claude-opus-4-6")
+    })
+
+    test("skips compaction agent messages", () => {
+      // given
+      const compactionMessage = {
+        agent: "compaction",
+        model: { providerID: "anthropic", modelID: "claude-opus-4-6" },
+      }
+      const validMessage = {
+        agent: "sisyphus",
+        model: { providerID: "anthropic", modelID: "claude-opus-4-6" },
+      }
+      writeFileSync(join(tempDir, "001.json"), JSON.stringify(compactionMessage))
+      writeFileSync(join(tempDir, "002.json"), JSON.stringify(validMessage))
+
+      // when
+      const result = findNearestMessageExcludingCompaction(tempDir)
+
+      // then
+      expect(result).not.toBeNull()
+      expect(result?.agent).toBe("sisyphus")
+    })
+
+    test("falls back to partial agent/model match", () => {
+      // given
+      const messageWithAgentOnly = {
+        agent: "hephaestus",
+      }
+      const messageWithModelOnly = {
+        model: { providerID: "openai", modelID: "gpt-5.3" },
+      }
+      writeFileSync(join(tempDir, "001.json"), JSON.stringify(messageWithModelOnly))
+      writeFileSync(join(tempDir, "002.json"), JSON.stringify(messageWithAgentOnly))
+
+      // when
+      const result = findNearestMessageExcludingCompaction(tempDir)
+
+      // then
+      expect(result).not.toBeNull()
+      // Should find the one with agent first (sorted reverse, so 002 is checked first)
+      expect(result?.agent).toBe("hephaestus")
+    })
+
+    test("returns null for empty directory", () => {
+      // given - empty directory (tempDir is already empty)
+
+      // when
+      const result = findNearestMessageExcludingCompaction(tempDir)
+
+      // then
+      expect(result).toBeNull()
+    })
+
+    test("returns null for non-existent directory", () => {
+      // given
+      const nonExistentDir = join(tmpdir(), "non-existent-dir-12345")
+
+      // when
+      const result = findNearestMessageExcludingCompaction(nonExistentDir)
+
+      // then
+      expect(result).toBeNull()
+    })
+
+    test("skips invalid JSON files and finds valid message", () => {
+      // given
+      const invalidJson = "{ invalid json"
+      const validMessage = {
+        agent: "oracle",
+        model: { providerID: "google", modelID: "gemini-2-flash" },
+      }
+      writeFileSync(join(tempDir, "001.json"), invalidJson)
+      writeFileSync(join(tempDir, "002.json"), JSON.stringify(validMessage))
+
+      // when
+      const result = findNearestMessageExcludingCompaction(tempDir)
+
+      // then
+      expect(result).not.toBeNull()
+      expect(result?.agent).toBe("oracle")
+    })
+
+    test("finds newest valid message (sorted by filename reverse)", () => {
+      // given
+      const olderMessage = {
+        agent: "older",
+        model: { providerID: "a", modelID: "b" },
+      }
+      const newerMessage = {
+        agent: "newer",
+        model: { providerID: "c", modelID: "d" },
+      }
+      writeFileSync(join(tempDir, "001.json"), JSON.stringify(olderMessage))
+      writeFileSync(join(tempDir, "010.json"), JSON.stringify(newerMessage))
+
+      // when
+      const result = findNearestMessageExcludingCompaction(tempDir)
+
+      // then
+      expect(result).not.toBeNull()
+      expect(result?.agent).toBe("newer")
+    })
+  })
+})