fix: assorted testing reliability fixes

[jaredm563]

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Summary

Testing fixes. No ticket/linear issue but these changes were prompted from discussions about CLI here:
https://netlify.slack.com/archives/C096B91TE03/p1769180475717899
https://netlify.slack.com/archives/C096B91TE03/p1769124255667039

Fix #1:
Changed test() to test.serial() for the "build plugins can manipulate netlifyToml.edge_functions array" test.

Both the default and tarball tests were running concurrently and writing to the same fixture directory :-/ this was causing some seemingly random failures since one test's bundle files would be overwritten by the other.

Fix #2:
Increased SERVER_POLL_TIMEOUT from 10s to 30s. The Deno server spinning up time in CI can be slower due to limited resources, causing the waitForServer() polling to timeout before the server is ready. This was causing flaky failures in server.test.ts.

Fix #3:
Added caching for downloaded Deno binaries (~/.netlify/deno-cli and ~/.netlify/deno-cli-v1) in the CI workflow
This is to prevent re-downloading Deno binaries on every CI run
Cache key includes a hash of bridge.ts to automatically invalidate when version changes.

Fix #4: Network Error Retries
Added p-retry dependency to netlify/config for retrying extension fetching on transient network errors. Follows existing pattern from packages/edge-bundler/node/downloader.test.ts which uses p-retry with vi.mock() to speed up retry tests by reducing timeouts from seconds to milliseconds.

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[github-actions]

This pull request adds or modifies JavaScript (.js, .cjs, .mjs) files.
Consider converting them to TypeScript.

[serhalp]

Great finds! I have some thoughts/questions inline