feat(environment): add prevent_self_review support

[gdams]

Summary

Adds support for the prevent_self_review setting on deployment environments.

When enabled, this prevents the user who triggered a deployment from being able to approve their own deployment when required reviewers are configured — a common compliance requirement for production environments.

API

The field is a top-level boolean on GitHub's Environment API payload (prevent_self_review), so it flows through otterdog's default from/to provider mappings without any extra transformation logic.

Example

environments+: [
  orgs.newEnvironment('production') {
    reviewers+: ['@my-org/platform-team'],
    prevent_self_review: true,
  },
],

Changes

• otterdog/models/environment.py — new prevent_self_review: bool field.
• examples/template/otterdog-defaults.libsonnet — default prevent_self_review: false on newEnvironment.
• otterdog/resources/schemas/environment.json — new boolean property.
• tests/models/test_environment.py + github-environment.json + otterdog-environment.json — updated to cover the new field.

Follow-ups (not in this PR)

Custom GitHub App deployment protection rules (managed via /repos/{owner}/{repo}/environments/{env}/deployment_protection_rules) require separate endpoints and a more complex model. Intentionally left as a separate change.

Testing

• poetry run pytest tests/ → 243 passed, 2 skipped, 0 failures
• poetry run mypy otterdog → clean
• poetry run ruff check / ruff format --check → clean

[netomi]

LGTM, can you add an entry to the CHANGELOG.md file?
It should also be documented in https://github.com/eclipse-csi/otterdog/blob/main/docs/reference/organization/repository/environment.md