test : add parser unit tests for secret_scanner plugin

[tmdeveloper007]

Summary of What Needs to be Done:
Add parser and contract unit tests for the secret_scanner plugin. The plugin parses Gitleaks JSON report output for secret/credential leak findings. Tests should cover normal JSON parsing, malformed JSON, empty output, and the executor normalization path.

Changes that Need to be Made:

• Create testing/backend/unit/test_secret_scanner_plugin.py following the existing plugin test pattern
• Create testing/backend/unit/fixtures/secret_scanner/sample_output.txt with Gitleaks JSON output
• Import the real parser from plugins/secret_scanner/parser.py
• Test: metadata loads through PluginManager validation path
• Test: build_command renders representative target
• Test: parser.parse on sample output produces expected findings
• Test: parser.parse on empty JSON array returns empty findings
• Test: parser.parse on malformed JSON is handled gracefully
• Test: executor._normalize_parsed_result on fixture output

Impact that it would Provide:

• Regression coverage for Gitleaks JSON output parsing
• Ensures credential leak findings are correctly extracted and surfaced
• Faster CI via targeted plugin allowlist

Note: This task is being handled by tmdeveloper007 — please assign to that account when picking it up.