chore: Enable parallel builds for coveralls

[kramaranya]

I noticed we get this error sometimes when running unit test CI job:

Error: Unprocessable Entity
Response: {"message":"Can't add a job to a build that is already closed. Build 17225808879 is closed. See docs.coveralls.io/parallel-builds","error":true}

Example from https://github.com/kubeflow/sdk/actions/runs/17225808879/job/48870011364
So I enabled parallel mode to upload coverage data for both Python versions simultaneously and keep the Coveralls build open
https://docs.coveralls.io/parallel-builds

/assign @andreyvelich @astefanutti @Electronic-Waste

[coveralls]

Pull Request Test Coverage Report for Build 17375061497

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 65.033%

---

Totals
Change from base Build 17349568399:	0.0%
Covered Lines:	292
Relevant Lines:	449

---

💛 - Coveralls

[andreyvelich]

Thank you @kramaranya!
/lgtm
/approve

[google-oss-prow]

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