feat: adding the reject method to task_updater

[MadaraUchiha-314]

Description

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• Adds a utility function to rejecting a task

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[holtskinner]

LGTM, @kthota-g Can you review?

[holtskinner]

@MadaraUchiha-314 Be sure to sign the CLA here:https://cla.developers.google.com/.

Then rescan the PR here: https://cla.developers.google.com/github/rescan?pr=google-a2a%2Fa2a-python%2Fpull%2F142&key=%01%40%A0%1F%11%F0E%C7%F1%28%1F%12%1D%BA%F9G8%98%00%1A%2A&expiry=2025-07-14

[MadaraUchiha-314]

closing in favor of: #147

I am not able to amend the commit info on this one. Kindly approve and merge the linked one ^

Thank you for reviewing!