ci: run code-mode unit tests on all bazel targets

[cconger]

Why

V8 should be stable under Bazel, so the codex-code-mode unit tests should run across the Bazel platform matrix. If these tests prove unstable, we should fix the tests rather than exclude them from CI.

What changed

• Remove the explicit //codex-rs/code-mode:code-mode-unit-tests exclusion from the macOS and Linux Bazel test jobs.
• Remove the same exclusion from the native Windows post-merge job.
• Keep the existing Windows gnullvm shard coverage.

Bazel test coverage

The target contains 26 unit tests. A fresh uncached local Bazel execution ran all 26 with 0 failures, 0 ignored tests, and 0 filtered tests.

PR Bazel CI selected the target on every enabled platform and reported a cached pass:

Platform	Passing CI job
macOS aarch64	Bazel test passed
macOS x86_64	Bazel test passed in 2.2s
Linux GNU	Bazel test passed in 0.4s
Linux musl	Bazel test passed in 0.4s
Windows gnullvm	Bazel test passed in shard 4/4 in 1.6s

[anp-oai]

Exciting! Would appreciate if you'd point a codex at monitoring main for flakes after this lands but otherwise lgtm!