Enforce strict default program switch policy

[mad-sol-dev]

Summary

• normalize program selection comparisons to prevent locked sessions from switching programs under strict policy
• ensure program switch policy defaults to strict when unset or invalid and add coverage for default behavior
• reset policy configuration in contract fixtures and verify unknown policies still return 400 on locked switches

Testing

• pytest bridge/tests/unit/test_program_context.py bridge/tests/contract/test_program_selection.py

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Codex Task