Support zero day cooldown

[v-robaiken]

Cooldown day values of 0 were silently dropped from the job payload. The fields were int with omitempty, so 0 was indistinguishable from unset:

before: {"cooldown":{}}
after:  {"cooldown":{"default-days":0,...}}

This makes 0 a usable way to explicitly disable cooldown.

Changes

• UpdateCooldown day fields are now *int — explicit 0 serializes, unset stays omitted.
• Added UpdateCooldown.Validate() in extractInput to reject negative values, covering all input paths (file, args, server, stdin) for update and graph.
• Tests for zero / non-zero / omitted across the JSON and YAML round-trips.

omitempty is retained per the model-versioning guidance in internal/model/job.go, so this stays compatible with dependabot-core before and after. No existing smoke tests set a zero cooldown.

[Copilot]

Pull request overview

Enables explicit zero-day cooldowns while rejecting negative values.

Changes:

• Uses pointer fields to distinguish zero from omitted values.
• Validates cooldown values before execution.
• Adds serialization and validation coverage.

Show a summary per file

File	Description
internal/model/job.go	Updates cooldown fields and validation.
internal/model/job_test.go	Tests serialization and validation.
internal/infra/run.go	Validates cooldowns before running jobs.
internal/infra/run_test.go	Tests run-parameter validation.
testdata/cooldown.yml	Adds a zero-day cooldown fixture.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0
• Review effort level: Balanced