Fix shared header encoding for update-with-start

[tconley1428]

Summary

• avoid mutating source header payloads while building encoded client requests
• add a regression test for an interceptor sharing a header across update-with-start inputs
• add an unreleased changelog entry

Fixes #1769

Validation

• poe test -s tests/worker/test_update_with_start.py -k shared_interceptor_header
• poe test -s tests/worker/test_workflow.py -k workflow_headers_with_codec
• poe lint