Mark snapshots missing backup as assist bases (#3618)

Loosen restrictions slightly by returning snapshots that are missing their bakcup or details models as assist bases that can be used by kopia assisted incrementals.

---

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

- [ ] 🌻 Feature
- [x] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Issue(s)

* #3525

#### Test Plan

- [ ] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E

src/internal/kopia/base_finder.go

@@ -192,6 +192,8 @@ func (b *baseFinder) findBasesInSet(
 					Manifest: man,
 					Reasons:  []Reason{reason},
 				})
+
+				logger.Ctx(ictx).Info("found incomplete backup")
 			}
 
 			continue
@@ -203,6 +205,18 @@ func (b *baseFinder) findBasesInSet(
 			// Safe to continue here as we'll just end up attempting to use an older
 			// backup as the base.
 			logger.CtxErr(ictx, err).Debug("searching for base backup")
+
+			if !foundIncomplete {
+				foundIncomplete = true
+
+				kopiaAssistSnaps = append(kopiaAssistSnaps, ManifestEntry{
+					Manifest: man,
+					Reasons:  []Reason{reason},
+				})
+
+				logger.Ctx(ictx).Info("found incomplete backup")
+			}
+
 			continue
 		}
 
@@ -216,6 +230,19 @@ func (b *baseFinder) findBasesInSet(
 				"empty backup stream store ID",
 				"search_backup_id", bup.ID)
 
+			if !foundIncomplete {
+				foundIncomplete = true
+
+				kopiaAssistSnaps = append(kopiaAssistSnaps, ManifestEntry{
+					Manifest: man,
+					Reasons:  []Reason{reason},
+				})
+
+				logger.Ctx(ictx).Infow(
+					"found incomplete backup",
+					"search_backup_id", bup.ID)
+			}
+
 			continue
 		}
 

src/internal/kopia/base_finder_test.go

@@ -436,6 +436,7 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 				1: testUser1Mail,
 			},
 			expectedAssistManifestReasons: map[int][]Reason{
+				0: testUser1Mail,
 				1: testUser1Mail,
 			},
 			backupData: []backupInfo{
@@ -470,6 +471,7 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 				1: testUser1Mail,
 			},
 			expectedAssistManifestReasons: map[int][]Reason{
+				0: testUser1Mail,
 				1: testUser1Mail,
 			},
 			backupData: []backupInfo{