Bun-native test runner support

[truthixify]

Tier: M (2-4 days) Type: test

Context

compat

Scope

Vitest works well in Node but Bun has its own test runner that's faster for CI. Confirm SDK tests run under bun test and add a CI matrix entry.

Acceptance criteria

• Verify all existing tests pass under bun test
• Document any incompatibilities (and fix where possible)
• Add bun-tests job to CI
• Note: vitest stays the primary runner; bun is supplemental

Files to start with

• Tests pass under both vitest and bun test
• CI matrix expanded
• Compatibility notes documented

Why this matters

• package.json
• .github/workflows/ci.yml