fix(servicebus): Preserve defaultMessageTimeToLive in subscription updates

[EldertGrootenboer]

Problem

ServiceBusAdministrationClient.updateSubscription() silently ignores changes to defaultMessageTimeToLive. The updated TTL value is discarded before XML serialization, causing the service to revert the property to its default (infinite TimeSpan). No error is thrown.

Fixes #48495

Root Cause

AdministrationModelConverter.getUpdateSubscriptionBody() nullifies defaultMessageTimeToLive alongside genuinely read-only properties (messageCount, createdAt, updatedAt, accessedAt, messageCountDetails, entityAvailabilityStatus) before serializing the update XML body. Since defaultMessageTimeToLive is a writable property that the user explicitly sets, it should not be nullified.

This regression was introduced in v7.17.5 by PR #42332 (which fixed UserMetadata being set to null on subscription update). The defaultMessageTimeToLive was accidentally included in the read-only property null-out block added by that PR.

Fix

Remove .setDefaultMessageTimeToLive(null) from the null-out chain in getUpdateSubscriptionBody(). The remaining null-outs for genuinely read-only properties are correct and unchanged.

This restores the pre-v7.17.5 behavior and aligns with how the .NET SDK handles subscription updates (SubscriptionPropertiesExtensions.cs), which serializes DefaultMessageTimeToLive as a writable field.

Testing

• Added updateSubscriptionPreservesDefaultMessageTimeToLive() unit test that verifies the converter preserves the TTL value in the update body.
• Full module test suite: 942 tests run, 0 failures, 0 errors.
• Formatting: spotless:check — 0 violations.
• Style: checkstyle:check — 0 violations.

[Copilot]

Pull request overview

Fixes a Service Bus administration regression where ServiceBusAdministrationClient.updateSubscription() dropped user-provided defaultMessageTimeToLive during update XML body generation, causing the service to keep/revert TTL unexpectedly.

Changes:

• Stop nullifying defaultMessageTimeToLive in AdministrationModelConverter#getUpdateSubscriptionBody().
• Add a regression unit test asserting the update body preserves defaultMessageTimeToLive.
• Document the bug fix in the module CHANGELOG.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
sdk/servicebus/azure-messaging-servicebus/src/main/java/com/azure/messaging/servicebus/administration/AdministrationModelConverter.java	Preserves defaultMessageTimeToLive when constructing the subscription update payload.
sdk/servicebus/azure-messaging-servicebus/src/test/java/com/azure/messaging/servicebus/administration/ServiceBusAdministrationClientTest.java	Adds a regression test validating TTL is not nullified in the update body.
sdk/servicebus/azure-messaging-servicebus/CHANGELOG.md	Adds an unreleased “Bugs Fixed” entry describing the TTL update fix.