Improve subdomain discovery confidence mapping

[mythri2405]

Description

This PR improves the subdomain_discovery plugin parser by standardizing how confidence and evidence metadata are represented for discovered subdomains. The goal is to make parser output more consistent, predictable, and aligned with existing plugin contract patterns.

The changes ensure that each finding exposes a clearer confidence mapping structure, improving downstream reliability and making the plugin easier to test and extend.
Minor updates were also made to unit tests to validate the updated parser behavior and ensure backward compatibility for existing discovery outputs.

Related Issues

Closes #859

Type of Change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Updated and extended tests in testing/backend/unit/test_subdomain_finder_plugin.py
• Verified parser output structure for multiple subdomain discovery formats
• Ensured confidence/evidence fields are consistently mapped across all findings
• Ran backend unit test suite to confirm no regression in existing plugin behavior

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• My changes generate no new warnings.