mgmt test, add BuildManager with defaultPollInterval and prepare release azure-resourcemanager-test

[XiaofeiCao]

Description

• fix mgmt test base, add buildManager(httppipeline, profile, pollInterval) override #49294
• tested locally with appservice
• prepare release

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• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
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[Copilot]

Pull request overview

Adds a new buildManager(Class, HttpPipeline, AzureProfile, Duration) overload to ResourceManagerTestProxyTestBase so ARM tests for "Lite"/generated packages can construct managers with a defaultPollInterval, reusing the already-built test HttpPipeline. Also dates and trims the 2.0.0-beta.3 CHANGELOG entry to prepare the release of azure-resourcemanager-test.

Changes:

• New reflective buildManager overload accepting a Duration defaultPollInterval for managers whose private constructor is (HttpPipeline, AzureProfile, Duration).
• CHANGELOG update: set release date 2026-06-01, remove empty sections, and document the new overload under Features Added.

Reviewed changes

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

File	Description
sdk/resourcemanager/azure-resourcemanager-test/src/main/java/com/azure/resourcemanager/test/ResourceManagerTestProxyTestBase.java	Adds reflective buildManager overload that locates and invokes the (HttpPipeline, AzureProfile, Duration) constructor on generated/Lite manager classes.
sdk/resourcemanager/azure-resourcemanager-test/CHANGELOG.md	Stamps 2.0.0-beta.3 with release date, drops empty subsections, and adds a Features Added entry describing the new helper.