feat(chat): strip reasoning when copying

Remove internal reasoning from copied message text to avoidleaking implementation details or developer-only when a
user content from the chat- In chat_pagecopyMessage, cleaning steps to:
  - <details type="ing">...</details blocks
  - <think>think> and <ing>...</reason> tags
 - trim leftover whitespace before writing to the clipboard
- In assistant_message_widget._buildSegmentedContent, remove an
  unused hasMediaAbove calculation and a conditional spacer that added
  extra top padding before reasoning tiles. This simplifies rendering
  logic and avoids relying on removed spacing behavior.

lib/features/chat/widgets/assistant_message_widget.dart

@@ -435,10 +435,6 @@ class _AssistantMessageWidgetState extends ConsumerState<AssistantMessageWidget>
 
   Widget _buildSegmentedContent() {
     final children = <Widget>[];
-    // Determine if media (attachments or generated images) is rendered above.
-    final hasMediaAbove =
-        (widget.message.attachmentIds?.isNotEmpty ?? false) ||
-        (widget.message.files?.isNotEmpty ?? false);
     bool firstToolSpacerAdded = false;
     int idx = 0;
     for (final seg in _segments) {
@@ -450,11 +446,6 @@ class _AssistantMessageWidgetState extends ConsumerState<AssistantMessageWidget>
         }
         children.add(_buildToolCallTile(seg.toolCall!));
       } else if (seg.isReasoning && seg.reasoning != null) {
-        // If a reasoning tile is the very first content and sits at the top,
-        // add a small spacer above it for breathing room.
-        if (children.isEmpty && !hasMediaAbove) {
-          children.add(const SizedBox(height: Spacing.sm));
-        }
         children.add(_buildReasoningTile(seg.reasoning!, idx));
       } else if ((seg.text ?? '').trim().isNotEmpty) {
         children.add(_buildEnhancedMarkdownContent(seg.text!));