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

[Nexussyn]

Closes #1

CONFIDENCE: 0.85
SOLUTION_SUMMARY: Add comprehensive diagnostic bundle validation tests that verify JSON/logd pairing, path normalization to repository-relative format, sensitive path redaction, and cross-platform deterministic behavior.
FILES_CHANGED:

• tests/test_diagnostic_bundle.py
• tests/conftest.py
  CODE:

# tests/conftest.py
import pytest
import os
import sys
import tempfile
import shutil

@pytest.fixture
def temp_diagnostic_dir():
    """Create a temporary diagnostic directory for testing."""
    temp_dir = tempfile.mkdtemp(prefix="diagnostic_test_")
    yield temp_dir
    shutil.rmtree(temp_dir, ignore_errors=True)

@pytest.fixture
def mock_build_context(temp_diagnostic_dir):
    """Provide a mock build context with known paths."""
    return {
        "diagnostic_dir": temp_di