Expose username validation evidence

[KingAkeem]

What changed

• Preserve the public X username-availability response alongside each confirmed scan result.
• Return the provider's validity flag, reason, message, and description as typed validation evidence.
• Add regression coverage for taken-username evidence.

Why

The scanner previously reduced the public response to a boolean. Keeping the non-secret validation evidence makes username results useful even when expanded X API profile inspection is unavailable or has depleted credits.

Validation

• Included in the platform test run: 12 backend tests passed.
• Confirmed the public availability endpoint reports the expected taken evidence for an existing account.