test(plugins): add malformed-output coverage for secret_scanner parser

[utksh1]

Summary

Add focused regression coverage for malformed, blank, and noisy output handled by plugins/secret_scanner/parser.py.

Scope

• Add or extend a focused backend parser test for this plugin.
• Exercise blank input, whitespace-only input, malformed records, and unrelated noise.
• Keep production parser behavior unchanged unless the test exposes a reproducible defect.

Acceptance criteria

• The parser does not raise for blank, whitespace-only, or malformed output.
• Valid records surrounding noisy lines still produce the expected findings.
• Assertions verify a stable, safe result shape for empty output.
• The targeted backend test passes.

Verification

Run the focused pytest file and the parser contract suite when applicable.