test : add parser unit tests for ssh_runner plugin

[tmdeveloper007]

Summary of What Needs to be Done:
Add parser and contract unit tests for the ssh_runner plugin. The plugin parses SSH command output to surface execution results. Tests should cover normal output, error indicators (Permission denied, Connection refused), empty output, and the executor normalization path.

Changes that Need to be Made:

• Create testing/backend/unit/test_ssh_runner_plugin.py following the existing plugin test pattern
• Create testing/backend/unit/fixtures/ssh_runner/sample_output.txt with representative SSH output
• Import the real parser from plugins/ssh_runner/parser.py
• Test: metadata loads through PluginManager validation path
• Test: build_command renders representative target
• Test: parser.parse on successful output produces one info finding
• Test: parser.parse on Permission denied output adjusts severity to medium
• Test: parser.parse on Connection refused output adjusts severity
• Test: parser.parse on empty output is deterministic
• Test: executor._normalize_parsed_result on fixture output

Impact that it would Provide:

• Regression coverage for SSH runner output parsing
• Ensures execution errors are correctly identified 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.

[aaniya22]

pls assign this to me under gssoc