feat: introducing w3c tracing headers propagation

[darklight3it]

Issue

N/A — W3C trace context propagation

Description

Adds a build-step middleware that propagates W3C trace context headers (traceparent, tracestate, baggage) from the Lambda InvokeStore onto outbound AWS SDK requests.

Why: When a Lambda function receives a request with trace context, downstream calls made via the AWS SDK should carry that context forward. This enables distributed tracing across service boundaries on the AWS SDK.

How it works:

• The middleware reads trace context from @aws/lambda-invoke-store (which the Lambda runtime populates per-invocation from inbound headers)
• If traceparent is present and the outgoing request doesn't already have one, it injects traceparent, tracestate, and baggage headers
• Existing trace headers are respected (never overwritten) and normalized to lowercase
• Browser and React Native get no-op implementations (InvokeStore is Node-only)

Design decisions:

• Registered as a plugin (getTraceContextPropagationPlugin) — not auto-added to generated clients.
• traceparent is the gate: tracestate and baggage are only propagated when traceparent is present (per W3C spec).
• InvokeStore.getInstanceAsync() is memoized at module level — subsequent calls on the hot path just await a resolved promise.

Testing

• 7 unit tests covering: full propagation, partial headers, missing traceparent, no InvokeStore context, existing headers preserved, case normalization, non-HTTP request no-op
• 1 integration test exercising a real S3 client middleware stack with requireRequestsFrom (no network calls)
• All tests pass locally. Fork CI cannot authenticate with upstream's AWS OIDC role (expected for fork PRs).

Checklist

• If the PR is a feature, add integration tests (*.integ.spec.ts) or E2E tests.
  • It's not a feature.
• My E2E tests are resilient to concurrent i/o.
  • I didn't write any E2E tests.
• I added access level annotations e.g. @public, @internal tags and enabled doc generation on the package. Remember that access level annotations go below the description, not above.
  • I didn't add any public functions.
• Streams - how do they work?? My WebStream readers/locks are properly lifecycled. Node.js stream backpressure is handled. Error handling.
  • No streams here.

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[kuhe]

moved to #8156