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

(reposting because of error in branch name for jira)

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Note

Low Risk
Low risk: a small signature/default-argument change plus a test update, with no change to adjoint computation logic beyond allowing omitted custom_vjp.

Overview
Adjoint postprocessing no longer requires custom_vjp. postprocess_adj now defaults custom_vjp to None, allowing callers (including backend imports) to omit the parameter.

Tests updated to prevent regression. The autograd emulation unit test stops passing custom_vjp=None explicitly, ensuring the defaulted call path is exercised.

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