Retry version check on transient proxy failures

[jojinkb]

Fixes #196

Problem

bootdev status — and the background update check that runs on every command via version.FetchUpdateInfo — intermittently fails with:

Unable to check version status
Error: failed to fetch latest version

The failure is transient (reproduces ~1 in 4 runs on my network). Instrumenting getLatestVersion shows the real cause is a dropped connection to the Go module proxy:

Get "https://proxy.golang.org/github.com/bootdotdev/bootdev/@latest": read tcp ...:443: read: connection reset by peer

getLatestVersion has no resilience to this:

• No retry — a single connection reset fails the whole check.
• No effective fallback — the proxy list is [proxy.golang.org, direct], but direct is (correctly) skipped since @latest can't be fetched from it over HTTP, so proxy.golang.org is effectively the only source.
• No timeout on http.Get, so a stalled connection can hang.
• The underlying error is swallowed and the HTTP status is never checked, so a non-200 body (or a 200 without a Version field) is indistinguishable from a real failure, and the user only ever sees the generic message.

Fix

• Retry each proxy up to 3 times with a short backoff before giving up — directly addresses the transient connection resets.
• Use an http.Client with a 10s timeout instead of bare http.Get.
• Check for HTTP 200 and reject empty-version responses.
• Wrap the underlying error into the returned one so bootdev status surfaces the real cause instead of an opaque message.

No change in behavior on the happy path; the version string returned is identical.

Tests

Added version/version_test.go (httptest-based, no network) covering:

• fetchLatestFromProxy: success, non-200, invalid JSON, empty version
• fetchLatestWithRetry: recovery after a transient dropped connection

go test ./... passes locally.