Add fail_fast option to policies and update related configurations

[dbrosio3]

This pull request introduces a new fail_fast option for built-in policy checks (diff_size and forbidden_paths), allowing configuration of whether a blocking violation should prevent later deterministic checks from running. The change is fully documented, validated, and tested across configuration, schema, runner logic, and documentation.

Key changes include:

Fail-fast policy configuration and behavior:

• Added a fail_fast boolean option to both diff_size and forbidden_paths policy configs, with schema and type updates (pushgate-config-v2.schema.json, src/config/types.ts, src/config/normalize.ts). This controls whether a blocking policy violation stops subsequent deterministic checks. [1] [2] [3] [4] [5] [6] [7]
• Updated the deterministic check runner to respect the fail_fast setting, so that if fail_fast is true and a blocking violation occurs, later checks are skipped. [1] [2] [3]

Documentation and configuration examples:

• Documented the new fail_fast option in docs/reference/configuration.md and added it to example YAML configs. [1] [2] [3] [4] [5] [6]

Testing and validation:

• Added and updated tests to verify correct parsing, normalization, runner behavior, and schema validation for the new fail_fast option, including explicit tests for fail-fast vs. aggregate behavior. [1] [2] [3] [4] [5] [6] [7]