android_local_test coverage reports only synthetic baseline data

Running `bazel coverage` on a Starlark `android_local_test` succeeds, but no execution coverage is collected.

I reproduced this on current `main` at `1d4cc58` with Bazel `9.0.0rc3`:

```sh
bazel coverage --combined_report=lcov --instrument_test_targets \
//test/rules/android_local_test/java/com/starlark_resources:no_deps_with_resources
```

The test passes, but its per-test `coverage.dat` is empty. The combined report contains only the synthetic baseline record:

```text
SF:test/rules/android_local_test/java/com/starlark_resources/SampleTest.java
FNF:0
FNH:0
LH:0
LF:0
end_of_record
```

`android_local_test` returns `InstrumentedFilesInfo`, but it does not define the implicit `_lcov_merger` dependency required for Bazel test rules. Bazel therefore runs the test with JaCoCo instrumentation, then `collect_coverage.sh` sees that `LCOV_MERGER` is unset and exits without processing the intermediate coverage data.

Bazel documents the required test rule setup here:
https://bazel.build/extending/rules#test-rules

Adding the coverage merger dependency to `android_local_test` produces line and function coverage in both the per-test and combined LCOV reports.

This is related to #558, but that issue covers broader extensibility for alternative coverage tools. This issue is about the default Bazel coverage path producing no execution data.

Start with the Starlark android_local_test rule and the test target //test/rules/android_local_test/java/com/starlark_resources:no_deps_with_resources. Trace how InstrumentedFilesInfo and the implicit _lcov_merger dependency are configured, then inspect collect_coverage.sh behavior when LCOV_MERGER is unset. Done means per-test coverage.dat and the combined LCOV report contain execution line and function coverage.

Written by the indexing model from the issue text.