🧪 Add unit tests for ChatViewModel attachment manipulation (#511)

* Add unit tests for ChatViewModel attachment manipulation

Adds tests covering addAttachment, removeAttachment (valid/invalid indices),
and clearAttachments in ChatViewModelTest. This ensures the attachment
state management logic works as expected.

* test(#511): add mixed valid/invalid remove sequence + DRY attachment setup

Folds in both Sourcery review notes:
- New testRemoveAttachment_mixedValidAndInvalidSequence asserts list
  stays consistent across a valid remove, out-of-bounds + negative no-ops,
  then another valid remove on the shifted list.
- addDummyAttachments() helper removes repeated VM/advance/add boilerplate.

Verified: ktlintCheck clean + ChatViewModelTest 37/37 pass locally.

app/src/test/java/com/m57/hermescontrol/ui/chat/ChatViewModelTest.kt

@@ -666,6 +666,127 @@ class ChatViewModelTest {
             }
         }
 
+    // ── Attachments ──────────────────────────────────────────────────────────
+
+    /** Add [count] dummy attachments so a test starts with a populated list. */
+    private fun TestScope.addDummyAttachments(
+        viewModel: ChatViewModel,
+        count: Int,
+    ) {
+        repeat(count) { i ->
+            viewModel.addAttachment("uri$i", "file$i.txt", "text/plain", (i + 1) * 100L)
+        }
+        advanceUntilIdle()
+    }
+
+    @Test
+    fun testAddAttachment() =
+        runTest {
+            val viewModel = createViewModel()
+            advanceUntilIdle()
+
+            viewModel.addAttachment(
+                uri = "content://dummy/1",
+                name = "dummy.txt",
+                mimeType = "text/plain",
+                size = 1024L,
+            )
+            advanceUntilIdle()
+
+            val pending = viewModel.uiState.value.pendingAttachments
+            assertEquals(1, pending.size)
+            assertEquals("content://dummy/1", pending[0].uri)
+            assertEquals("dummy.txt", pending[0].name)
+        }
+
+    @Test
+    fun testRemoveAttachment_validIndex() =
+        runTest {
+            val viewModel = createViewModel()
+            advanceUntilIdle()
+
+            viewModel.addAttachment("uri1", "file1.txt", "text/plain", 100)
+            viewModel.addAttachment("uri2", "file2.txt", "text/plain", 200)
+            advanceUntilIdle()
+
+            assertEquals(2, viewModel.uiState.value.pendingAttachments.size)
+
+            viewModel.removeAttachment(0)
+            advanceUntilIdle()
+
+            val pending = viewModel.uiState.value.pendingAttachments
+            assertEquals(1, pending.size)
+            assertEquals("uri2", pending[0].uri)
+        }
+
+    @Test
+    fun testRemoveAttachment_invalidIndex() =
+        runTest {
+            val viewModel = createViewModel()
+            advanceUntilIdle()
+
+            viewModel.addAttachment("uri1", "file1.txt", "text/plain", 100)
+            advanceUntilIdle()
+
+            assertEquals(1, viewModel.uiState.value.pendingAttachments.size)
+
+            // Out of bounds index should not crash or change list
+            viewModel.removeAttachment(5)
+            viewModel.removeAttachment(-1)
+            advanceUntilIdle()
+
+            val pending = viewModel.uiState.value.pendingAttachments
+            assertEquals(1, pending.size)
+            assertEquals("uri1", pending[0].uri)
+        }
+
+    @Test
+    fun testRemoveAttachment_mixedValidAndInvalidSequence() =
+        runTest {
+            val viewModel = createViewModel()
+            advanceUntilIdle()
+            addDummyAttachments(viewModel, 3) // [uri0, uri1, uri2]
+
+            // 1. Remove a valid index (the middle item) → list shrinks correctly.
+            viewModel.removeAttachment(1)
+            advanceUntilIdle()
+            assertEquals(2, viewModel.uiState.value.pendingAttachments.size)
+            assertEquals("uri0", viewModel.uiState.value.pendingAttachments[0].uri)
+            assertEquals("uri2", viewModel.uiState.value.pendingAttachments[1].uri)
+
+            // 2. Fire invalid removals (out of bounds + negative) — must be no-ops.
+            viewModel.removeAttachment(99)
+            viewModel.removeAttachment(-1)
+            advanceUntilIdle()
+            assertEquals(2, viewModel.uiState.value.pendingAttachments.size)
+
+            // 3. Another valid removal on the shifted list → still consistent.
+            viewModel.removeAttachment(1)
+            advanceUntilIdle()
+            val pending = viewModel.uiState.value.pendingAttachments
+            assertEquals(1, pending.size)
+            assertEquals("uri0", pending[0].uri)
+        }
+
+    @Test
+    fun testClearAttachments() =
+        runTest {
+            val viewModel = createViewModel()
+            advanceUntilIdle()
+
+            viewModel.addAttachment("uri1", "file1.txt", "text/plain", 100)
+            viewModel.addAttachment("uri2", "file2.txt", "text/plain", 200)
+            advanceUntilIdle()
+
+            assertEquals(2, viewModel.uiState.value.pendingAttachments.size)
+
+            viewModel.clearAttachments()
+            advanceUntilIdle()
+
+            val pending = viewModel.uiState.value.pendingAttachments
+            assertTrue(pending.isEmpty())
+        }
+
     // ── Send message ─────────────────────────────────────────────────────────
 
     @Test