fix(adjoint): optional argument for postprocess_adj for backend

[groberts-flex]

Optional argument for when this is imported by the backend for the adjoint pipeline

updated emulated_run in unit test to catch a regression for this

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Note

Low Risk
Small signature change with a safe default; risk is limited to potential downstream callers that relied on the parameter being required.

Overview
Allows backend integrations to call postprocess_adj without providing a custom_vjp hook by making the parameter optional (default None).

Updates the autograd emulation unit test to omit custom_vjp=None, ensuring the backward pass still succeeds when the argument is not supplied, and records the fix in the changelog.

^{Written by Cursor Bugbot for commit 6d97fbd. This will update automatically on new commits. Configure here.}

[groberts-flex]

wrong branch name!

[github-actions]

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