docs: fix OTel vendor extension key references

[narutamaaurum]

Summary

• replace stale extensions["x.otel"] references in the OTel middleware TSDoc
• align the tracing example comments/README with the actual x-vendor.opentelemetry.tracecontext key
• keep the docs/examples consistent with the runtime constant and package docs

Validation

• git diff --check
• rg "x\\.otel" /root/work/arcp-ts-sdk-84 (no matches)
• full workspace lint/typecheck/test not run in this runner because the repo requires Node >=22 while this environment has Node v12.22.9

Closes #84

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