fix: align schedule API responses with contract

[hugobrenet]

• Make schedule responses deterministic by sorting entries by object, node, key, action, and schedule.
• Return the documented 400 response for invalid object kinds and 404 for unknown objects.
• Serialize unset run timestamps as null and empty item collections as [].
• Align the OpenAPI schema with runtime responses by fixing the ScheduleItem kind and declaring timestamps as nullable.
• Properly propagate peer error responses and close proxied response bodies.
• Share schedule item construction across node and instance endpoints and add regression tests.

[aikido-pr-checks]

🟡 Medium - Object schedule silently drops peers with no local instance

When the aggregate's replicated instance-config cache still lists a node whose local object configuration has already disappeared, that peer returns 404 No local instance (Trigger). The new branch treats every peer 404 as an empty contribution (Mechanism). The object endpoint then returns 200 with a partial schedule list and gives callers no indication that one configured instance was omitted (Consequence).

Show fix

Only ignore the peer's explicitly expected No schedule table cached response, or otherwise verify that the 404 represents a configured-but-unscheduled instance; propagate No local instance (and other missing-instance responses) so callers do not receive a falsely complete aggregate.

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