[Bug] Can we make 'schedule' field optional when schedule: false

**Describe the bug**
When creating dbt jobs through the console it is possible to not enable a schedule. When creating jobs through the dbt-jobs-as-code cli this is a mandatory field, even when it is not used.

**To Reproduce**
Steps to reproduce the behavior:
1. Create a new job
2. Disable trigger-schedule
3. add schedule: NULL or no schedule at all
4. dbt-jobs-as-code validate flags this job and it won't deploy

```yml
# yaml-language-server: $schema=https://raw.githubusercontent.com/dbt-labs/dbt-jobs-as-code/main/src/dbt_jobs_as_code/schemas/load_job_schema.json

jobs:
manual_job_example:
account_id: 123
environment_id: 456
project_id: 789
name: "Manual Job - No Schedule"
execute_steps:
- "dbt build --select my_model"
triggers:
schedule: false
```

**Expected behavior**
It would be nice if it is not necessary to fill in this unnecessary information.

**Desktop (please complete the following information):**
- macOs
- dbt Cloud CLI - 0.40.14 (09a03c3022aa6c7db909ab4d265ef832373b314f 2026-01-07T17:25:23Z)
- dbt-jobs-as-code, version 1.15.1