[JainAyush-01]

This Pull request:

Fixes an in-place string mutation bug in TPluginManager::LoadHandlersFromPluginDirs that breaks plugin discovery when Root.PluginPath (or ROOT_PLUGIN_PATH) contains multiple colon-separated directories.

Changes or fixes:

Root Cause:

In commit e60640c (#18859), gSystem->ConcatFileName was replaced with gSystem->PrependPathName. Unlike ConcatFileName which returned a new heap-allocated string without modifying the input, PrependPathName(const char *dir, TString &name) modifies name in place.

In TPluginManager::LoadHandlersFromPluginDirs(const char *base):

TString sbase = base;
...
for (Int_t i = 0; i < dirs->GetEntriesFast(); i++) {
   ...
   if (!skip) {
      if (sbase != "") {
         const char *p = gSystem->PrependPathName(d, sbase);
         LoadHandlerMacros(p);
      }
   }
}

Because sbase was passed directly to PrependPathName without copying to a temporary variable:

• On iteration 0 (dir0 = "/custom/plugins"): sbase was mutated from "TArchiveFile" to "/custom/plugins/TArchiveFile".
• On iteration 1 (dir1 = "$ROOTSYS/etc/plugins"): PrependPathName prepended dir1 to the already mutated sbase, resulting in "$ROOTSYS/etc/plugins//custom/plugins/TArchiveFile".

As a result, whenever a user or experiment environment (e.g. ALICE O2 / JAliEn) prepended a custom directory to Root.PluginPath, all subsequent directories—including ROOT's own built-in plugins in $ROOTSYS/etc/plugins—failed to load.

Fix:

Copy sbase to a temporary TString temp = sbase; inside the loop before calling PrependPathName(d, temp), matching the pattern already used in the else branch (lines 545–546) and throughout TPluginManager.cxx.

Testing:

• Added GoogleTest regression test TEST(TPluginManager, MultiDirectoryPluginPath) in core/base/test/TPluginManagerTests.cxx.
• Verified that both custom plugins in a prepended directory and built-in plugins in subsequent directories are resolved properly.
• Ran test via ctest -R CorePluginManagerTests: 100% passed (0.26s).

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR fixes #23536