[Sushisource]

What was changed

• Add explicit HeaderEncodingDataConverter, requiring combined converter/codec capability; ordinary CodecDataConverter constructors and defaults are unchanged.
• Apply codec-only header processing at shared client/worker boundaries, with SDK wrapper forwarding, contextual validation, shared start/update decoding, and tracing serialization preserved.
• Preserve schedule header round trips, activity header-presence behavior, child destination namespace, and external-storage ordering; avoid duplicate update-with-start processing.
• Resolve/cache derived codecs once in consuming client constructors and use one recursive header-chain validation pass. Add focused tests, documentation, and changelog notes.

Breaking changes: None. Custom outer converter wrappers must forward the optional header capability or keep the header-encoding converter outermost.
Server PR: Not required.

Why?

Part of temporalio/features#850: normalize opt-in processing of user header payloads without reserializing Payload objects or changing existing defaults.

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:
   • Focused SDK tests across 11 test classes, retained signed-payload workflow tests, and four OpenTracing test classes passed; one existing external-service-only memo test was skipped.
   • Validation compiled/tested the changed sources using a scratch Gradle init override with read-only protobuf sources at the repository's exact pinned revisions. Submodule-update tasks and spotlessApply were excluded because the local proto/protocloud directories are partial.
   • Canonical no-override validation was not run for the final cleanup. Sequential cached Google Java Format 1.24 checks left five discrepancies outside the cleanup hunks unchanged; that CLI check did not use the earlier long-string-reflow exemption.
   • Filename/package/public-type and conflict-marker audits, git diff --check, and byte comparison of the merged files against the tested snapshot passed.
   • Full repository tests and dedicated old-history replay fixtures were not run.
3. Any docs updates needed? README, public API documentation, and changelog included; migration guidance covers pre-codec header payloads, legacy decoders, and wrapper forwarding.