Add smoke test workspace and pipeline for testing Bazel CI changes (#2796)

This PR adds a minimal, self-contained smoke test workspace and a
corresponding Buildkite pipeline in the `bazel-testing` org to enable
testing Bazel CI script changes across all supported platforms without
colliding on a shared branch.

### Changes
1. **Smoke Test Workspace** (`smoke_test/`):
- `smoke_test/MODULE.bazel`: Bzlmod configuration with `rules_cc`
dependency.
- `smoke_test/BUILD.bazel`: Defines a minimal `:hello` binary and
`:hello_test` test target.
- `smoke_test/hello.cc` & `smoke_test/hello_test.cc`: C++ binary and
test sources.
- `smoke_test/smoke_test.yml`: Task matrix covering all 27 supported
platforms.
2. **Terraform Pipeline**
(`buildkite/terraform/bazel-testing/pipelines_misc.tf`):
- Added `buildkite_pipeline.smoke-test` which runs `project_pipeline
--file_config=smoke_test/smoke_test.yml`.
- Configured with `filter_condition = "build.pull_request.labels
includes \"CI:run\" || build.branch !~ /.+:.+/"`.
3. **`.gitignore`**:
- Added `/smoke_test/bazel-*` to ignore Bazel symlinks in `smoke_test/`.