feat: make schedule optional for ci and merge jobs

[b-per]

Summary

• Makes the schedule block optional for ci and merge job types, since a cron schedule is not functionally relevant for those jobs
• Adds if/then/else conditional to the JSON schema so editor validation (yaml-language-server) also enforces the conditional requirement
• Adds a CI test to catch stale JSON schemas when models change

Closes #190

Test plan

• Pydantic validation: ci/merge jobs work without schedule, scheduled/other jobs still require it
• JSON schema validation: mirrors the same conditional logic
• All 138 tests pass
• Existing configs with schedule provided continue to work for all job types

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
src/dbt_jobs_as_code
main.py	310	160	48%	120–123, 129, 168–169, 200–203, 209, 245–329, 400–405, 418–419, 437–438, 442–444, 461–510, 539–597, 628–673, 678–679, 683
src/dbt_jobs_as_code/client
__init__.py	179	60	66%	16, 53–54, 62, 89–104, 119–120, 139–140, 147–157, 170–171, 187–188, 205, 215, 245–253, 295–310, 333, 349–350, 360–372, 375–386, 391–400
src/dbt_jobs_as_code/cloud_yaml_mapping
change_set.py	235	30	87%	27–29, 59, 65–66, 155, 212, 224, 247, 251–252, 258–260, 275–278, 334–348, 401–423, 459–473
src/dbt_jobs_as_code/importer
__init__.py	27	7	74%	14–15, 22–27
src/dbt_jobs_as_code/schemas
__init__.py	23	2	91%	68, 78
common_types.py	60	3	95%	67–68, 91
config.py	14	1	93%	18
job.py	130	6	95%	227, 232, 248–257
TOTAL	1180	269	77%

Tests	Skipped	Failures	Errors	Time
138	0 💤	0 ❌	0 🔥	13.083s ⏱️

[b-per]

CI passed 🚀