test(storage-incentives): add synctest to TestAgent

[akrem-chabchoub]

Checklist

• I have read the coding guide.
• My change requires a documentation update, and I have done it.
• I have added tests to cover my changes.
• I have filled out the description and linked the related issues.

Description

The main issue was in the agent test where inconsistent timing would sometimes cause the test to fail.
This has been addressed by using synctest to wrap code that will create a bubble to help preventing timing issues.

Testing

• Update test commands in Makefile:
  $(GO) test -count=1 -run "TestAgent" github.com/ethersphere/bee/v2/pkg/storageincentives

• Check Attempts: https://github.com/ethersphere/bee/actions/runs/18543306676

Open API Spec Version Changes (if applicable)

Motivation and Context (Optional)

Related Issue (Optional)

#5232

Screenshots (if appropriate):

[Copilot]

Pull Request Overview

This PR addresses timing issues in the storage incentives agent test by integrating Go's testing/synctest package to create deterministic test execution. The main change wraps the test logic in synctest.Test() to prevent flaky test failures caused by inconsistent timing.

• Adds synctest integration to create deterministic test timing
• Includes additional validation to ensure expected contract calls occur
• Removes unnecessary empty line for code cleanup

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[Copilot]

This validation contradicts the test logic above. When tc.expectedCalls is false (lines 129-135), the test expects no calls and fails if any are found. Adding this check means tests with expectedCalls: false will always fail since they expect zero calls but this code requires at least one call.