Fix stack frame filtering to use exact path comparison

[Copilot]

The update_stack_frames method filtered debugger frames using path.include?(__FILE__), which incorrectly filters application code when file paths contain the debugger path as a substring.

Changes:

• Replace substring match with exact path comparison: path == @debugger_file
• Cache File.expand_path(__FILE__) as instance variable to avoid repeated filesystem calls

Before:

locations.each do |loc|
  path = File.expand_path(loc.path)
  next if path.include?(__FILE__)  # Matches "/app/debugger.rb_utils.rb"
  filtered << loc
end

After:

# In initialize
@debugger_file = File.expand_path(__FILE__)

# In update_stack_frames
locations.each do |loc|
  path = File.expand_path(loc.path)
  next if path == @debugger_file  # Only matches exact debugger file
  filtered << loc
end

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