Fix accessibility: message composer label and 'Seen by' button (#6390, #6393)

- MarkdownTextInput: set contentDescription on MarkdownEditText so TalkBack
  announces a persistent label even when the field has content (fixes #6390)
- TimelineItemReadReceiptView: remove hideFromAccessibility() so the 'Seen by'
  button is exposed to TalkBack; use semantics (not clearAndSetSemantics) to
  preserve click action, and add Role.Button (fixes #6393)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/components/receipt/TimelineItemReadReceiptView.kt

@@ -25,9 +25,10 @@ import androidx.compose.ui.Modifier
 import androidx.compose.ui.draw.clip
 import androidx.compose.ui.res.pluralStringResource
 import androidx.compose.ui.res.stringResource
-import androidx.compose.ui.semantics.clearAndSetSemantics
+import androidx.compose.ui.semantics.Role
 import androidx.compose.ui.semantics.contentDescription
-import androidx.compose.ui.semantics.hideFromAccessibility
+import androidx.compose.ui.semantics.role
+import androidx.compose.ui.semantics.semantics
 import androidx.compose.ui.semantics.testTag
 import androidx.compose.ui.tooling.preview.PreviewParameter
 import androidx.compose.ui.unit.dp
@@ -57,18 +58,12 @@ fun TimelineItemReadReceiptView(
     modifier: Modifier = Modifier,
 ) {
     if (state.receipts.isNotEmpty()) {
-        ReadReceiptsRow(
-            modifier = modifier.clearAndSetSemantics {
-                hideFromAccessibility()
-            }
-        ) {
+        ReadReceiptsRow(modifier = modifier) {
             ReadReceiptsAvatars(
                 receipts = state.receipts,
                 modifier = Modifier
                     .clip(RoundedCornerShape(4.dp))
-                    .clickable {
-                        onReadReceiptsClick()
-                    }
+                    .clickable { onReadReceiptsClick() }
                     .padding(2.dp)
             )
         }
@@ -137,9 +132,10 @@ private fun ReadReceiptsAvatars(
     val receiptDescription = computeReceiptDescription(receipts)
     Row(
         modifier = modifier
-            .clearAndSetSemantics {
+            .semantics {
                 testTag = TestTags.messageReadReceipts.value
                 contentDescription = receiptDescription
+                role = Role.Button
             },
         horizontalArrangement = Arrangement.spacedBy(4.dp - avatarStrokeSize),
         verticalAlignment = Alignment.CenterVertically,