Ensure kv collection refresh settings are not considered unless the feature is enabled.

[jimmyca15]

This PR does the following:

• Ensure that kv collection refresh interval is not used unless collection based refresh of key-values is enabled.
• Fix short circuit code path when no refresh is due.
• Add tests to ensure that minimum refresh interval is respected for key-values and feature flags.

Impact wise, these changes should have little to no behavioral/performance impact. Mostly code clarity.

In response to #632

PR Overview

This PR ensures that the key-value collection refresh interval is only applied when collection‑based refresh is enabled and adds tests to verify that both key-value and feature flag refresh intervals are respected.

• Added tests to confirm that the refresh interval for a single key and feature flags is enforced.
• Updated the refresh logic in the provider to check that kv collection refresh settings are only used when RegisterAll is enabled.
• Added a precondition check to throw an exception when an invalid KvCollectionRefreshInterval is provided.

Reviewed Changes

File	Description
tests/Tests.AzureAppConfiguration/RefreshTests.cs	Added tests for ensuring minimum refresh intervals and verifying that refresh calls behave as expected.
src/Microsoft.Extensions.Configuration.AzureAppConfiguration/AzureAppConfigurationProvider.cs	Updated refresh logic to consider RegisterAllEnabled and introduced a validation check for KvCollectionRefreshInterval.

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

Comments suppressed due to low confidence (1)

src/Microsoft.Extensions.Configuration.AzureAppConfiguration/AzureAppConfigurationProvider.cs:121

• [nitpick] Consider enhancing the exception message to more explicitly state that the KvCollectionRefreshInterval is only applicable when using RegisterAll enabled mode.

if (options.KvCollectionRefreshInterval <= TimeSpan.Zero)

[jimmyca15]

@amerjusupovic it looks like the tests i added already existed. I removed them. But I noticed that tests were using Thread.Sleep, I replaced them with Task.Delay.