fix(trace): resolve stack trace duplication in output

[usernane]

Description

Stack trace entries appear duplicated in error output. Visible in test output where same entries show twice.

Steps to Reproduce

1. Run composer test
2. Observe stack trace output showing duplicate entries like:

#0 At class CLIFormattingTest line 118
#1 At class CLIFormattingTest line 126
...
#12 At class CLIFormattingTest line 118
#13 At class CLIFormattingTest line 126

Implementation Details

• Review AbstractHandler::setTrace() logic
• Review ErrorHandlerException::getDebugTrace()
• Ensure trace is built once, not concatenated multiple times
• Add test to verify no duplicate entries

Acceptance Criteria

• Stack trace shows each entry exactly once
• Test added to prevent regression