Fix concurrent issue during bundle file concurrent generation testing

[JohnNiang]

What type of PR is this?

/kind failing-test
/area core
/milestone 2.17.x

What this PR does / why we need it:

I wrongly invoked Arraylist#add(probes) method in multi threads. So the unit test was unstable and might encounter the problem as follows:

Expected :1
Actual   :0
<Click to see difference>

org.opentest4j.AssertionFailedError: expected: <1> but was: <0>
	at org.junit.jupiter.api.AssertionFailureBuilder.build(AssertionFailureBuilder.java:151)
	at org.junit.jupiter.api.AssertionFailureBuilder.buildAndThrow(AssertionFailureBuilder.java:132)
	at org.junit.jupiter.api.AssertEquals.failNotEqual(AssertEquals.java:197)
	at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:166)
	at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:161)
	at org.junit.jupiter.api.Assertions.assertEquals(Assertions.java:632)
	at run.halo.app.core.extension.service.impl.PluginServiceImplTest$BundleCacheTest.concurrentComputeBundleFileIfAbsent(PluginServiceImplTest.java:460)

See https://github.com/halo-dev/halo/actions/runs/9382059472/job/25832681545 for more.

This PR moves the invocation outside thread tasks.

Does this PR introduce a user-facing change?

None

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.2% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.47%. Comparing base (5fdf6c0) to head (b98e785).
Report is 227 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6043      +/-   ##
============================================
- Coverage     56.91%   56.47%   -0.44%     
- Complexity     3319     3541     +222     
============================================
  Files           587      622      +35     
  Lines         18968    20961    +1993     
  Branches       1401     1478      +77     
============================================
+ Hits          10795    11838    +1043     
- Misses         7594     8523     +929     
- Partials        579      600      +21     

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[ruibaby]

/lgtm

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ruibaby

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[JohnNiang]

/cherry-pick release-2.16

[halo-dev-bot]

@JohnNiang: new pull request created: #6045

In response to this:

/cherry-pick release-2.16

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