[TT-17356] Add W3C trace context extract/inject helpers for body-carried propagation

[lghiur]

Adds two helpers to the trace package for cases where the W3C trace context travels in a request body (e.g. MCP/JSON-RPC params._meta) rather than the HTTP traceparent header:

• ExtractW3CTraceContext(ctx, traceParent, traceState) — installs a W3C traceparent/tracestate as the active remote span context so later spans join that trace. Always W3C, regardless of the configured propagator.
• CurrentW3CTraceContext(ctx) — serialises the active span context back to W3C traceparent/tracestate (empty when no span is active). Inverse of the above.

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[lghiur]

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