Add support for message schedules

[mtmk]

This pull request introduces support for the new AllowMsgSchedules property in the JetStream StreamConfig, enabling the scheduling of messages as described in ADR-51. It also adds a corresponding test to verify correct behavior when creating and updating streams with this property.

JetStream StreamConfig enhancements:

• Added the AllowMsgSchedules boolean property to the StreamConfig class, allowing message scheduling to be enabled or disabled for streams.

Testing improvements:

• Introduced a new test, AllowMsgSchedules_property_should_be_set_on_stream, which verifies that the AllowMsgSchedules property is correctly set, persisted, and enforces that disabling it after enabling returns the expected error.
• Added missing import for NATS.Client.TestUtilities in ManageStreamTest.cs to support the new test.

[Copilot]

Pull Request Overview

This PR adds support for message scheduling in JetStream by introducing the AllowMsgSchedules property to the StreamConfig class, implementing the new feature described in ADR-51.

• Added AllowMsgSchedules boolean property to StreamConfig with proper JSON serialization attributes
• Created comprehensive test coverage to verify property setting, persistence, and error handling when disabling schedules

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/NATS.Client.JetStream/Models/StreamConfig.cs	Added AllowMsgSchedules property with JSON serialization configuration
tests/NATS.Client.JetStream.Tests/ManageStreamTest.cs	Added test method and required import to verify message scheduling functionality

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[scottf]

LGTM