fix: support file paths with spaces in block headers (#1504)

The block header pattern used %S+ (non-whitespace characters) to match
file paths, which failed when paths contained spaces. This prevented
code blocks from being parsed and made diff acceptance (<C-y>)
non-functional for files in directories with spaces.

Changes:
- Add new pattern with lazy match (.-) to support paths with spaces
- Keep original pattern as fallback for compatibility
- Pattern now correctly parses headers like:
  python path=/path/with spaces/file.py start_line=1 end_line=10

Fixes the issue where blocks count remained 0 even when Treesitter
correctly identified block_header and block_content nodes.

Tested with paths containing spaces and verified that:
- Blocks are now correctly parsed and added to message.section.blocks
- Diff acceptance mappings (accept_diff, show_diff, etc.) now work
- Backward compatibility maintained for paths without spaces

Author: junqizhang

lua/CopilotChat/ui/chat.lua

@@ -39,6 +39,7 @@ local function match_block_header(header)
   end
 
   local patterns = {
+    '^(%w+)%s+path=(.-)%s+start_line=(%d+)%s+end_line=(%d+)$',
     '^(%w+)%s+path=(%S+)%s+start_line=(%d+)%s+end_line=(%d+)$',
     '^(%w+)$',
   }