fix(ci): generate JUnit XML to silence codecov-action warnings (#300)

## Summary

`getsentry/codecov-action@main` has `enable-tests: true` by default,
searching for JUnit XML files matching `**/*.junit.xml`. Since the
project produced no JUnit output, every CI run emitted 3 warnings about
missing JUnit XML files.

Bun's test runner natively supports JUnit XML via `bunfig.toml`, so
enabling it is the cleanest fix — it also unlocks test result reporting
(test counts, pass rate, test deltas vs base branch) in the codecov
action for free.

## Changes

- **`bunfig.toml`**: added `[test.reporter]` section with `junit =
"test-results.junit.xml"`
- **`.gitignore`**: added `*.junit.xml` under test artifacts so the
generated file isn't accidentally committed

No CI workflow changes needed — the codecov action's default glob
`**/*.junit.xml` already matches the output path.

Author: BYK

.gitignore

@@ -16,6 +16,7 @@ coverage
 
 # test artifacts
 .test-tmp
+*.junit.xml
 
 # logs
 logs