test: add validation tests for readBoulderState session_ids handling

Add tests for corrupted/incomplete boulder.json:
- null JSON value returns null
- primitive JSON value returns null
- missing session_ids defaults to []
- non-array session_ids defaults to []
- empty object defaults session_ids to []
- appendSessionId with missing session_ids does not crash

Refs #1672

src/features/boulder-state/storage.test.ts

@@ -43,6 +43,78 @@ describe("boulder-state", () => {
       expect(result).toBeNull()
     })
 
+    test("should return null for JSON null value", () => {
+      //#given - boulder.json containing null
+      const boulderFile = join(SISYPHUS_DIR, "boulder.json")
+      writeFileSync(boulderFile, "null")
+
+      //#when
+      const result = readBoulderState(TEST_DIR)
+
+      //#then
+      expect(result).toBeNull()
+    })
+
+    test("should return null for JSON primitive value", () => {
+      //#given - boulder.json containing a string
+      const boulderFile = join(SISYPHUS_DIR, "boulder.json")
+      writeFileSync(boulderFile, '"just a string"')
+
+      //#when
+      const result = readBoulderState(TEST_DIR)
+
+      //#then
+      expect(result).toBeNull()
+    })
+
+    test("should default session_ids to [] when missing from JSON", () => {
+      //#given - boulder.json without session_ids field
+      const boulderFile = join(SISYPHUS_DIR, "boulder.json")
+      writeFileSync(boulderFile, JSON.stringify({
+        active_plan: "/path/to/plan.md",
+        started_at: "2026-01-01T00:00:00Z",
+        plan_name: "plan",
+      }))
+
+      //#when
+      const result = readBoulderState(TEST_DIR)
+
+      //#then
+      expect(result).not.toBeNull()
+      expect(result!.session_ids).toEqual([])
+    })
+
+    test("should default session_ids to [] when not an array", () => {
+      //#given - boulder.json with session_ids as a string
+      const boulderFile = join(SISYPHUS_DIR, "boulder.json")
+      writeFileSync(boulderFile, JSON.stringify({
+        active_plan: "/path/to/plan.md",
+        started_at: "2026-01-01T00:00:00Z",
+        session_ids: "not-an-array",
+        plan_name: "plan",
+      }))
+
+      //#when
+      const result = readBoulderState(TEST_DIR)
+
+      //#then
+      expect(result).not.toBeNull()
+      expect(result!.session_ids).toEqual([])
+    })
+
+    test("should default session_ids to [] for empty object", () => {
+      //#given - boulder.json with empty object
+      const boulderFile = join(SISYPHUS_DIR, "boulder.json")
+      writeFileSync(boulderFile, JSON.stringify({}))
+
+      //#when
+      const result = readBoulderState(TEST_DIR)
+
+      //#then
+      expect(result).not.toBeNull()
+      expect(result!.session_ids).toEqual([])
+    })
+
     test("should read valid boulder state", () => {
       // given - valid boulder.json
       const state: BoulderState = {
@@ -129,6 +201,23 @@ describe("boulder-state", () => {
       // then
       expect(result).toBeNull()
     })
+
+    test("should not crash when boulder.json has no session_ids field", () => {
+      //#given - boulder.json without session_ids
+      const boulderFile = join(SISYPHUS_DIR, "boulder.json")
+      writeFileSync(boulderFile, JSON.stringify({
+        active_plan: "/plan.md",
+        started_at: "2026-01-01T00:00:00Z",
+        plan_name: "plan",
+      }))
+
+      //#when
+      const result = appendSessionId(TEST_DIR, "ses-new")
+
+      //#then - should not crash and should contain the new session
+      expect(result).not.toBeNull()
+      expect(result!.session_ids).toContain("ses-new")
+    })
   })
 
   describe("clearBoulderState", () => {