Run the Fastify e2e suite on Bun and Deno from one app

[JPeer264]

Run the Fastify e2e suite on Node, Bun and Deno from one app, following #24598 (react-router-8-framework). Node stays the required job. Bun and Deno start as optional variants and become required once green.

Depends on #24598 (runtime matrix key, runtimeEntryPlugin, README convention) and #24612 (Bun diagnostics channels, missing http.server span).

Apps

• Base: node-fastify-5
• New variants: bun, deno

Fastify does not run on Cloudflare Workers (see the audit cell below), so there is no Cloudflare variant. Record that decision here.

Cover on every runtime

The existing suite runs unchanged and branches on RUNTIME only where runtimes are expected to differ. Add assertions where the suite has none today:

• Errors
• Parameterized route names
• Per-request isolation
• Incoming trace continuation
• Outgoing fetch with sentry-trace and baggage
• Logs

Findings to confirm (runtime audit, September 2026, develop 6f2da7483d; database spans were measured with mysql or mysql2):

• Bun: db spans: partial.
• Deno: outgoing fetch: broken. deno: Outgoing fetch gets no span and no trace propagation #23891 has added outgoing fetch spans to @sentry/deno since.
• Cloudflare: does not run.

Setups the audit ran: Bun, Deno, Cloudflare

Done when

• node-fastify-5 runs its suite on Node, Bun and Deno
• Every variant is green and required, or this issue records why a runtime is not supported
• SDK defects the variants surface are fixed here, or split into their own issue when the fix is large or shared with other libraries

[linear-code]

JS-3849