Export QueryAssure results as JUnit XML

[Victoria824]

Why

JUnit XML would let existing CI systems display QueryAssure cases alongside ordinary test results without a custom integration.

Acceptance criteria

• Add an optional --junit <path> output to the test command
• Map each contract to one testcase with stable suite/case names
• Represent blocked regressions as failures with the gate and concise reason
• Escape untrusted content and exclude result rows, secrets, and credentials
• Add round-trip XML parsing tests and a README example

Helpful starting points

• src/queryassure/cli.py
• src/queryassure/reporting.py
• tests_py/test_core.py