CI: Extract unit tests into independent workflow

[chargome]

Linear ticket: https://linear.app/getsentry/issue/JS-1879

Depends on: #19677 (build output caching)
Parent: #19671

Goal

Extract unit test jobs from build.yml into an independent ci-unit-tests.yml workflow. This includes:

• job_browser_unit_tests
• job_node_unit_tests (×4 Node versions)
• job_bun_unit_tests
• job_deno_unit_tests

This is the last extraction since unit tests are the least flaky and fastest to complete, so re-run wait times are least painful.

New workflow: ci-unit-tests.yml

name: 'CI: Unit Tests'
on:
  push:
    branches: [develop, master, v9, v8, release/**]
  pull_request:
  merge_group:
    types: [checks_requested]
  workflow_dispatch:

Jobs

1. restore_build — uses .github/actions/restore-build (from CI: Build output caching + cross-workflow restore-cache action #19677)
2. browser_unit_tests — Vitest browser tests
3. node_unit_tests — matrix: Node 18/20/22/24
4. bun_unit_tests — conditional on @sentry/bun changes
5. deno_unit_tests — conditional on @sentry/deno changes
6. unit_tests_gate — gate job named Unit Tests Passed

Conditional execution

Bun and Deno tests use changed_bun and changed_deno flags. This workflow needs the same detection.

Transition steps

1. Create ci-unit-tests.yml. Unit tests run in both build.yml and ci-unit-tests.yml (dual-running).
2. Verify stability over several days/PRs.
3. Ruleset change (requires admin): Add Unit Tests Passed as required status check to all 4 rulesets.
4. Remove unit test jobs from build.yml and from job_required_jobs_passed needs list.
5. At this point build.yml only contains build, lint, and check jobs. Rename workflow to CI: Build & Lint and gate job to Build & Lint Passed. Update rulesets accordingly.

Concurrency

concurrency:
  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
  cancel-in-progress: true

[linear-code]

JS-1879