fix: validate job description max length of 255 characters

[b-per]

Summary

• Adds max_length=255 to the description field on JobDefinition, giving users a clear Pydantic validation error instead of a silent HTTP 500 from the API
• Updates the generated JSON schema (load_job_schema.json) with maxLength: 255
• Adds tests covering the boundary (255 accepts, 256 rejects) for both Pydantic and JSON schema validation

Closes #204

Root cause

The dbt Cloud API enforces a 255-character limit on job descriptions but returns a generic 500 error with no useful message. The limit is not documented in the API spec.

Test plan

• uv run pytest tests/schemas/test_job.py::TestDescriptionValidation — all 4 tests pass
• uv run pytest — full suite (171 tests) passes
• Manually verified against the API: 255-char description succeeds, 256-char description fails with 400

[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	46	74%	16, 53–54, 62, 99–100, 119–120, 139–140, 155–156, 170–171, 187–188, 205, 215, 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	133	7	95%	237, 242, 258–268
TOTAL	1184	256	78%

Tests	Skipped	Failures	Errors	Time
171	0 💤	0 ❌	0 🔥	13.414s ⏱️

[b-per]

CI passed 🚀