bun: CI pins a single Bun version

`.github/workflows/build.yml:522` pins `bun-version: '1.3.14'` with no matrix, while Node runs a `[20.19, 22, 24, 26]` matrix. Bun ships breaking runtime changes often, and several of the findings above turn on Bun internals (`Module.registerHooks` presence, diagnostics-channel support, `Bun.serve` option shape).

**Work item.** Add a two-entry matrix: the pinned version plus `latest`, with `continue-on-error` on `latest` so it reports drift without blocking.

**Prior art ***(related history only)*. getsentry/sentry-javascript#20890 (merged) is the PR that pinned Bun to 1.3.14. The drift it hides is already visible: getsentry/sentry-javascript#23521 (merged) "test(bun): Fix integration test on Bun 1.4.0" fixed a break found outside CI. No issue tracks adding a matrix.

Start at .github/workflows/build.yml:522 and compare the Bun job with the existing Node version matrix. Add the pinned Bun version and latest as the two matrix entries, with latest allowed to fail without blocking. Confirm the workflow reports both Bun versions and that failures on latest do not fail the required CI run.

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