Label validator for target name allows space in name

### Description of the bug:

Target name according to https://github.com/bazelbuild/bazel/blob/c32926f879b4378c7df8e54c93b00f1a7feef901/site/en/concepts/labels.md?plain=1#L139 can not have spaces.
Yet logic in https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/cmdline/LabelValidator.java#L39-L69 allows that

### Which category does this issue belong to?

_No response_

### What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

_No response_

### Which operating system are you running Bazel on?

Linux, Mac OS

### What is the output of `bazel info release`?

release 8.4.1

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

```text

```

### If this is a regression, please try to identify the Bazel commit where the bug was introduced with bazelisk --bisect.

_No response_

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

_No response_

Start at src/main/java/com/google/devtools/build/lib/cmdline/LabelValidator.java, especially lines 39-69, and compare its target-name validation with the linked labels documentation. Trace the existing label-validation tests or entry point before changing behavior; done means target names containing spaces are rejected consistently with the documented syntax and a regression case covers it.

Written by the indexing model from the issue text.