[Sushisource]

What was changed

• Add default-off WorkerOptions.codecHeaders to decode incoming Workflow/Activity headers and encode headers emitted by Workflows, including tracing headers and nested System Nexus payload headers.
• Leave client-supplied Payload headers wire-ready. There is no converter-wide or client header-encoding option, and no client/schedule/converter source changes in this PR.
• Add focused boundary tests, worker documentation, and a changelog fragment.

Breaking changes: None; existing worker header behavior is the default.
Server PR: Not required.

Why?

A .NET client with a codec can write an encoded tracing header that the TS worker's OpenTelemetry interceptor cannot read because worker header decoding is skipped. This worker-only option fixes that boundary without double-encoding user-provided client Payloads.

Checklist

1. Related issues: [Feature Request] Run headers through codec #514 and Normalize Header Data Conversion across languages features#850.
2. How was this tested:
   • Cross-language evidence: features regression and driver, commit 5f040bd. Real .NET 1.21.1 → this TS implementation passes with codecHeaders: true: encoded _tracer-data verified in history, input/result round-trip, caller trace identity preserved, and no failed Workflow Tasks. No temporary SDK patch or decoding interceptor workaround was used. TS 1.25.0 reproduces Unknown encoding: binary/encrypted with the same feature.
   • 41 focused tests passed: workflow header opt-in/out, activity ingress, nested System Nexus headers, and wire-ready client headers across starts, retained schedules, and update-with-start retries.
   • Worker/dependency build, focused test compilation, formatting, and changelog validation passed. ESLint reported no errors and three warnings in unchanged client code.
   • The full test-package build remains blocked by OpenTelemetry dependency-type conflicts in tracing test files under unlocked dependency resolution; full suites and remote CI were not verified. The pre-existing local lockfile edit was not changed or committed.
3. Any docs updates needed? Worker README, API comments, and changelog included. Client headers should already contain any codec encoding; workflow-produced headers are encoded by the worker only when this option is enabled.