fix(tracing): tracing capability check

[ben-reitz]

Fixes #2087 for agents on older workerd versions that had tracing defined but did not have a startActiveSpan function.

   Old workerd                    → silently use a no-op tracer
     tracing is undefined

   Intermediate workerd           x throws an error
     tracing.enterSpan exists
     tracing.startActiveSpan does not exist

   Modern workerd                 → use native Cloudflare tracing already
     tracing.enterSpan exists
     tracing.startActiveSpan exists

This fix also handles the intermediate case where a partial tracing runtime exists.

[changeset-bot]

🦋 Changeset detected

Latest commit: 90288ae

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
agents	Patch
@cloudflare/agent-think	Patch

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

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commit: 90288ae