fix(test): fix flaky platform/fabric/services/endorser

[mbrandenburger]

TestParallelCollectEndorsementsOnProposalView_TimesOutOnSilentParty was flaky because parallelCollectEndorsementsOnProposalView.Call armed its aggregate deadline with the same duration that collectEndorsement passes to ReceiveWithTimeout, so both fired at once and select picked between them at random — one path reports "time out reached on session", the other "timeout waiting for endorsement", and only the first matched the assertion.

The aggregate deadline now runs a fixed grace period longer than the per-party timeout, so a silent party is consistently reported by the error that names it, leaving the aggregate deadline as the backstop for the session-setup and send steps that nothing else bounds.

Along the way this fixes a real bug in the same loop: the AppendProposalResponse error path wrapped a.err, which is always nil there, so Wrapf returned nil and Call handed the caller (nil, nil) instead of the append failure. Adds tests for the aggregate deadline and for the swallowed append error, and extracts the fake wiring the parallel-endorsement tests share into a fixture.

[github-actions]

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[SaidAltury-ibm]

LGTM thank you @mbrandenburger

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