fix(deno): instrument outgoing fetch requests

[psang39]

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This adds tracing support for outgoing fetch requests in the Deno SDK, based on the existing Bun fetch integration.

The new integration:

• creates http.client spans for outgoing fetch requests
• propagates sentry-trace and baggage headers according to tracePropagationTargets
• skips requests sent to Sentry
• leaves fetch breadcrumb recording to the existing breadcrumbs integration, avoiding duplicate breadcrumbs
• is enabled by default and exported for manual configuration

Tests cover default integration registration, span creation, trace-header propagation, and preservation of exactly one fetch breadcrumb.

Testing

• corepack yarn lint in packages/deno — passed
• oxfmt --check on the changed files — passed
• deno test --allow-net --allow-read --allow-run --allow-env --no-check test/deno-fetch.test.ts — 2 passed
• Full corepack yarn test in packages/deno — 66 passed, 1 failed in the existing captureException path-normalization assertion on Windows (C:/... vs app:///...)

Closes #23891