[Sushisource]

What was changed

• Fix the CI-exposed activity shutdown continuation race: termination waiting now observes the existing shutdown chain before resource waits, within the same timeout budget and without blocking later forced-shutdown escalation.
• 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:
   • Converter opt-in/delegation, contextual codec validation, and SDK-wrapper forwarding.
   • Header propagation through client/worker boundaries, tracing, storage, schedules, and multioperations.
   • Shutdown-continuation, timeout/interruption, and graceful-to-forced escalation regressions.
3. Any docs updates needed? README, public API documentation, and changelog included; migration guidance covers pre-codec header payloads, legacy decoders, and wrapper forwarding.