Fix rust_test does not emit junit XML to bazel XML_OUTPUT_FILE

[peakschris]

Add JUnit XML test reporting to rust_test rules. When bazel test runs a Rust test, the wrapper parses libtest output and writes JUnit XML to $XML_OUTPUT_FILE for CI integration (Jenkins, GitHub Actions, etc.). Zero overhead on bazel run - the wrapper execs the test binary directly when $XML_OUTPUT_FILE is unset. Opt out per-target with junit = False.

Fixes #1303

Related: #3948, #3490

Changes

• rust/private/rust.bzl: Add junit attribute (default True) and _junit_runner to rust_test. When enabled, symlink the runner as the executable and set RUST_TEST_BIN pointing to the real test binary via rlocation.
• util/junit_runner/: New Rust binary that wraps test execution, parses libtest stdout, and writes JUnit XML.
• util/collect_coverage/collect_coverage.rs: When the junit wrapper is active, resolve the instrumented binary from RUST_TEST_BIN instead of TEST_BINARY. Also use COVERAGE_OUTPUT_FILE when set.

Test plan

• //util/junit_runner:junit_runner_test - 14 unit tests for parsing and XML generation
• //test/junit:junit_test - end-to-end: wrapper runs test successfully
• //test/junit:no_junit_test - opt-out with junit = False
• bazel test //... - no regressions (1 pre-existing failure in remap_path_prefix on Windows, unrelated)

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