Fix: [$25 BOUNTY] [Python] Add diagnostic bundle validation tests

[Nexussyn]

Closes #1

CONFIDENCE: 0.75

SOLUTION_SUMMARY: Added comprehensive diagnostic bundle validation tests that verify JSON/logd contract integrity, path redaction, repository-relative paths, and cross-platform determinism without relying on live services.

FILES_CHANGED:

• tests/test_diagnostic_bundle.py
• tests/conftest.py
• diagnostic/.gitkeep

CODE:

### FILE: tests/test_diagnostic_bundle.py
"""
Diagnostic bundle validation tests.

This module provides deterministic tests that validate:
- Matching JSON and .logd file pairs
- Repository-relative path formatting
- Redaction of sensitive local paths and usernames
- Proper failure modes for missing/mismatched artifacts
"""

import json
import os
import re
import sys
import tempfile
import platform
from pathlib import Path
from typing import Dict,