feat: show policy approvers in PR comments

[edbighead]

Summary

When a conftest policy check fails, Atlantis posts a GitHub comment with a "Policy Approval Status" section but doesn't indicate who is allowed to approve — forcing contributors to hunt through Atlantis config to find the right team or person to contact.

This PR surfaces the configured owner teams and users directly in the PR comment for each failing policy set:

• Adds OwnerUsers []string and OwnerTeams []string to PolicySetResult
• Populates them in conftest_client.go and project_command_runner.go by merging global (PolicySets.Owners) and per-set owners with case-insensitive deduplication
• Renders approver info in both wrapped and unwrapped policy check comment templates — only when owners are configured, so existing behaviour is unchanged when no owners are set

Example output (when owners are configured):

#### Policy Approval Status:
...
* :white_check_mark: To **approve** this project, comment:
  ```shell
  atlantis approve_policies ...

• common-policies approval required from: teams: infra-team users: alice

## Test plan

- [x] Existing unit tests pass: `go test ./server/events/... ./server/core/runtime/policy/...`
- [x] Updated `conftest_client_test.go` expected JSON strings to include new fields
- [x] Template change is backward-compatible: no output change when `OwnerUsers` and `OwnerTeams` are both empty

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