Make *_property tasks idempotent

[josegonzalez]

The shared setProperty / unsetProperty helpers in tasks/properties.go (line 66 and line 109) carry // todo: validate that the value isn't already set/unset comments. They currently always exec dokku <plugin>:set and unconditionally report Changed: true on success. As a result, every property task built on executeProperty (builder_property, git_property, network_property, nginx_property, scheduler_property) is non-idempotent.

Implement idempotency in the shared helper:

• Query current value via dokku <plugin>:report <app> --<plugin>-<property> (per-app) or dokku <plugin>:report --<plugin>-global-<property> (global). Info-flag form is required because --format json is unsupported by git:report / nginx:report, and rejected entirely for any --global query (see dokku/dokku follow-ups).
• On present: if current value matches desired, return Changed: false, State: "present" without exec.
• On absent: if current value is empty, return Changed: false, State: "absent" without exec.
• On unknown property (stderr matches Invalid flag passed, valid flags:): warn via log.Printf and fall through to exec.
• On any other report error (dokku missing, app missing, not deployed, transient failure): silent fallthrough to exec — preserves today's behavior.
• The set/unset exec error path is unchanged: TaskOutputErrorFromExec continues to wrap stderr/err into the returned state.

Tests should cover:

• Repeated present of the same value reports Changed: false.
• Repeated absent reports Changed: false.
• Unknown property triggers the warning and still falls through to exec.
• Existing validation tests continue to pass.

Spun off from #128 to keep that PR scoped to mirroring the existing property-task pattern.