Allow platforms with slashes in their names in output paths (#31600)

### Description

With `--experimental_use_platforms_in_output_dir_legacy_heuristic` (the default), the name of the target platform becomes the first component of the output directory mnemonic. Slashes in that name are now replaced with underscores, so `platform(name = "foo/bar")` results in `bazel-out/foo_bar-fastbuild` instead of failing analysis.

This can make two platforms share an output directory, but that is already the case for platforms with the same name in different packages. Falling back to the `platform-<hash>` form used without the legacy heuristic would avoid that, but would also make the output directory unreadable for these platforms.

### Motivation

Target names may contain slashes, but such platforms failed analysis with `CPU/Platform descriptor 'foo/bar' is invalid as part of a path: must not contain /`. Since `--experimental_platform_in_output_dir` defaults to `auto` in Bazel 9, this affects every exec configuration with such an execution platform.

Fixes #31596

### Build API Changes

No

### Release Notes

RELNOTES: Platforms whose names contain `/` no longer fail analysis when used in output directory names.

Closes #31600

COPYBARA_INTEGRATE_REVIEW=https://github.com/bazelbuild/bazel/pull/31600 from fmeum:platform-name-with-slash 1b9488416872279f7eb403cef61acc3adf986120
PiperOrigin-RevId: 996792452
Change-Id: I3eac1a916aade404eefdec50336c5a7a930a9684

src/main/java/com/google/devtools/build/lib/analysis/config/OutputPathMnemonicComputer.java

diff --git a/src/main/java/com/google/devtools/build/lib/analysis/config/OutputPathMnemonicComputer.java b/src/main/java/com/google/devtools/build/lib/analysis/config/OutputPathMnemonicComputer.java
index 00aca42..269df35 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/config/OutputPathMnemonicComputer.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/config/OutputPathMnemonicComputer.java
@@ -275,7 +275,8 @@
       // Only use non-default platforms.
 
       if (!PlatformOptions.platformIsDefault(platform)) {
-        return platform.getName();
+        // Target names may contain slashes, but the mnemonic has to be a single path segment.
+        return platform.getName().replace('/', '_');
       }
       // Fall back to using the CPU.
       ctx.markAsExplicitInOutputPathFor("cpu");

src/test/java/com/google/devtools/build/lib/analysis/config/BuildConfigurationValueTest.java

diff --git a/src/test/java/com/google/devtools/build/lib/analysis/config/BuildConfigurationValueTest.java b/src/test/java/com/google/devtools/build/lib/analysis/config/BuildConfigurationValueTest.java
index 8b6a2ef..03d5753 100644
--- a/src/test/java/com/google/devtools/build/lib/analysis/config/BuildConfigurationValueTest.java
+++ b/src/test/java/com/google/devtools/build/lib/analysis/config/BuildConfigurationValueTest.java
@@ -834,6 +834,18 @@
   }
 
   @Test
+  public void testPlatformInOutputDir_legacy_withSlashInPlatformName() throws Exception {
+    scratch.file("platform/BUILD", "platform(name = 'alpha/beta')");
+    BuildConfigurationValue config =
+        create(
+            "--experimental_use_platforms_in_output_dir_legacy_heuristic",
+            "--platforms=//platform:alpha/beta");
+
+    assertThat(config.getOutputDirectory().getRoot().toString())
+        .matches(".*/[^/]+-out/alpha_beta-fastbuild");
+  }
+
+  @Test
   public void testPlatformInOutputDir_defaultPlatform() throws Exception {
     BuildConfigurationValue config =
         create(