test: add automated test coverage for bundles and policy edge cases (closes #2)

[ghzhost]

Summary

Closes #2.

Adds deterministic automated test coverage in tests/test_behaviors_extended.py covering key bundle loading, strict prompt template validation, source catalog action filtering, and improvement policy gate lifecycle edge cases.

Changes

• test_load_bundle_unknown_raises_value_error: verifies that attempting to load a non-existent behavior bundle raises ValueError.
• test_render_prompt_unknown_prompt_id_raises_value_error: validates that referencing an undefined prompt name in a bundle raises ValueError.
• test_render_prompt_extra_variables_raises_value_error: ensures strict template rendering rejects extraneous parameters.
• test_source_catalog_rejects_empty_or_unmatched_targets: verifies actions with unauthorized/unmatched targets are rejected.
• test_improvement_policy_reverts_on_insufficient_scores_after_gate_limit: verifies failing candidate exhaustion triggers revert.
• test_improvement_policy_continues_when_insufficient_before_limit: verifies candidate remains in isolation when gate count is below limit.

Verification

• PYTHONPATH=backend:. uv run pytest -q passes (80 passed).
• uv run ruff check orbit/ backend/ tests/ passes cleanly.
• Tests are fully deterministic and require no external credentials or network access.