[Sushisource]

What was changed

• Assign the configured client converter as a fallback for value-backed update and update-with-start headers.
• Preserve explicitly supplied header converters and prebuilt Payload entries without decoding/reencoding them.
• Add one parameterized regression test covering both paths and converter choices, plus README guidance.

Breaking changes: None; explicit converters and raw payload forwarding are unchanged.
Server PR: Not required.

Why?

Part of temporalio/features#850. Unlike workflow start, these update paths omitted converter assignment, so headers created from ordinary PHP values without an explicit converter could fail serialization. PHP uses its existing converter extension point; no separate codec abstraction is introduced.

Checklist

1. Related issue: Normalize Header Data Conversion across languages features#850 (this PR does not close the cross-SDK issue).
2. How was this tested:
   • php -l passed for both changed implementation files and tests/Unit/Client/UpdateHeaderTestCase.php.
   • git diff --check passed; independent source review found no issues.
   • PHPUnit, coding-style checks, and Psalm could not run because Composer and vendor/ are unavailable. These checks remain unverified.
3. Any docs updates needed? README guidance included. This repository has no changelog-fragment convention, so no new changelog infrastructure was introduced.