[FR]: Change exit code when linting requested but no lintable targets provided

### What is the current behavior?

When running `aspect lint` and providing an explicit list of targets, the process will exit with 1 if no lint-able targets were specified.

### Describe the feature

The request here is to change the exit code to 4, [matching the behaviour of `bazel test`](https://bazel.build/run/scripts) when testing is requested but no test targets are specified.

Original slack thread: https://bazelbuild.slack.com/archives/C04281DTLH0/p1784137418806149

Run `aspect lint` with an explicit target list containing no lintable targets and compare the result with `bazel test` when no test targets are specified. Trace the lint command's exit-code handling and add or update a regression test so the no-lintable-target case exits 4 while normal lint behavior remains unchanged.

Written by the indexing model from the issue text.