Fix ObjectDisposedException when opening Net OS Heap Alloc Stacks

[Copilot]

Issue

When users try to open Net OS Heap Alloc Stacks view in PerfView, they encounter an ObjectDisposedException with the message:

Cannot access a disposed object. Object name: 'NativeSymbolModule'.

Root Cause

GetAllocationType method caches NativeSymbolModule objects in the loadedModules dictionary, but the SymbolReader class has its own limited cache (size 10) and disposes of modules when they're evicted from this cache. This creates a stale reference problem where the cached NativeSymbolModule objects in loadedModules are disposed but still referenced.

Fix

Changed the approach to cache PDB file paths rather than NativeSymbolModule objects:

1. Modified the loadedModules dictionary from:

   var loadedModules = new Dictionary<TraceModuleFile, NativeSymbolModule>();

   to:

   var loadedModules = new Dictionary<TraceModuleFile, string>(); // Cache PDB paths, not NativeSymbolModule objects

2. Updated the caching logic in GetAllocationType:

   // Get the PDB path from cache or find it
   if (!loadedModules.TryGetValue(module, out var pdbPath))
   {
       pdbPath = (module.PdbSignature != Guid.Empty
           ? symReader.FindSymbolFilePath(module.PdbName, module.PdbSignature, module.PdbAge, module.FilePath)
           : symReader.FindSymbolFilePathForModule(module.FilePath));
       loadedModules[module] = pdbPath; // Cache the path, not the module
   }
   
   // Get a fresh NativeSymbolModule for each lookup
   var symbolModule = (pdbPath != null) ? symReader.OpenNativeSymbolFile(pdbPath) : null;

This ensures we always get a fresh NativeSymbolModule from SymbolReader.OpenNativeSymbolFile each time, allowing the SymbolReader to properly manage its cache and object lifetimes.

Fixes #2199.

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