Summary

• The trojansource plugin (B613) crashed with FileNotFoundError when bandit reads from stdin (bandit -)
• The plugin opened context.filename directly, but for stdin input the filename is the sentinel string ''
• Fixed by using context.file_data (the already-opened binary stream) instead of re-opening the file
• This also avoids an unnecessary second file open for regular files

Fixes #1182

Test plan

• Existing test_trojansource and test_trojansource_latin1 tests pass
• Full functional test suite passes (79/79)
• Manual test: cat examples/trojansource.py | bandit - now correctly detects bidi characters instead of crashing
• Manual test: echo 'print("hello")' | bandit - works cleanly with no errors
• ruff check and ruff format pass