deno: Outgoing `fetch` gets no span and no trace propagation

[isaacs]

This is the highest-impact finding in the audit. fetch is the idiomatic Deno HTTP client, and distributed tracing out of a Deno service is broken.

deno/src/integrations/breadcrumbs.ts:50 calls addFetchInstrumentationHandler, but only to record breadcrumbs. Nothing in packages/deno/src calls instrumentFetchRequest. Node has nativeNodeFetchIntegration, Bun has fetchIntegration, Cloudflare has fetchIntegration, vercel-edge has winterCGFetchIntegration. Deno has none.

Deno also emits no diagnostics channel for fetch (verified: node emits undici:request:create, deno and bun emit nothing), so denoHttpIntegration does not cover it either.

Probe result, running the built @sentry/deno under deno 2.9.2:

sentry-trace sent? false
baggage sent?     false
transactions: [ "GET /downstream", "parent" ]
  child spans: []   <- no http.client span under "parent"
  child spans: []

The downstream request became a separate root transaction instead of a child of parent, which is exactly the symptom a user would report as "my traces are broken".

Work item. Add a fetchIntegration to @sentry/deno modeled on packages/bun/src/integrations/fetch.ts (which is already a near-copy of the Cloudflare one) and put it in the default set. Add an integration test that asserts sentry-trace and baggage reach the downstream server and that an http.client child span is produced.

**Prior art **(related history only). None. The nearest work is #21009 (merged) "feat(deno): instrument node:http on versions that support it", which covered the node:http client and left fetch untouched. The integration to copy is #22869 (merged), the Bun fetch integration. Given fetch is Deno's primary HTTP client, the absence of any report suggests nobody has looked, not that nobody is affected.

[linear-code]

JS-3517