Finish collapsing pkg/procmgr/rust onto Bazel

### What does this PR do?
Remove the `procmgr_rust_tests` GitLab job and its entry from
`JOBOWNERS`, and add the `rust_test` target this uncovered was missing
for `dd-procmgr`.

### Motivation
Investigating why `rawzip` bump (#53563) broke CI found
`procmgr_rust_tests` failing because Cargo's internal registry proxy had
not yet mirrored the new version, not because of anything wrong with the
bump.
The same registry-lag failure independently hit `uuid` bump (#53552), a
dependency this crate uses directly.

Bazel benefits from ADMS' centralized downloader mirror config
(`.adms/bazel/adms.mirror.cfg`), while plain `cargo` instead relies on
`.rust_internal_registry`'s bespoke GitLab CI redirect
(`.adms/rust/gitlab.yaml`), which is what lagged, so `bazel:test:*`
passed both times regardless.

This is safe to rely on because `pkg/procmgr/rust/BUILD.bazel` has a
`rust_test` target (since #46880) and `bazel:test:linux-amd64`,
`:linux-arm64`, etc. already run every target in that file, with the
same implied `rustfmt`/`clippy` `-Dwarnings` Cargo passes explicitly, on
every pipeline.

The one gap, caught by review on #53629, was that `cargo test
--all-targets` also exercised `dd-procmgr.rs`'s own 15 `#[test]`
functions via `--bins`, which no existing `rust_test` covered; this
closes it with a `dd-procmgr_test` target wrapping that binary crate
directly.

The Cargo job also runs far more than its own path filter suggests:
`.on_main_or_release_branch` matches on branch name alone, ahead of the
path filter, so it fires on every push to `main`, ~900 times in the last
30 days for an average of ~140s per run.

As #53615 already dropped the `extend` from the package-build jobs,
removing it here finishes collapsing `pkg/procmgr/rust` onto Bazel as
its only toolchain, one source of truth instead of two that are doomed
to drift apart.

### Describe how you validated your changes
Confirmed parity before removing anything: no test is gated by the
`test-helpers` feature beyond `cfg(test)`, and `rules_rust`'s
`clippy`/`rustfmt` aspects don't propagate through dependencies, so they
only check whatever `//...` names directly, which is every library,
binary, and test as its own target.
Ran `bazel test //pkg/procmgr/rust:dd-procmgr_test --test_output=all`,
confirming all 15 tests run and pass.

### Additional Notes
Bazel's coverage is now a strict superset: it lints the plain and
`test-helpers`-enabled lib variants as separate targets and tests the
`dd-procmgr` CLI binary directly, whereas Cargo's single invocation only
ever builds one feature resolution at a time.

Author: rdesgroppes

.gitlab/JOBOWNERS

@@ -33,7 +33,6 @@ tests_serverless-init*                  @DataDog/serverless-azure-gcp
 security_go_generate_check              @DataDog/agent-security
 prepare_sysprobe_ebpf_functional_tests* @DataDog/ebpf-platform
 prepare_secagent_ebpf_functional_tests* @DataDog/agent-security
-procmgr_rust_tests                      @DataDog/agent-runtimes
 test_kmt_local_setup*                   @DataDog/ebpf-platform
 
 # Send count metrics about Golang dependencies

.gitlab/build/source_test/procmgr.yml

@@ -177,6 +177,14 @@ rust_test(
     ],
 )
 
+rust_test(
+    name = "dd-procmgr_test",
+    crate = ":dd-procmgr",
+    edition = "2024",
+    rustc_flags = ["--cfg=bazel"],
+    target_compatible_with = _LINUX_OR_WINDOWS,
+)
+
 rust_test(
     name = "e2e_test",
     srcs = [