Log SDK environment variable messages only when values differ (#12918)

### Context

Structured log viewer was cluttered with repeated non-actionable
messages about SDK environment variables (e.g., `DOTNET_HOST_PATH`).
These appeared for every project evaluation, reporting normal SDK
behavior without providing value to users.

### Changes Made

Modified `ProjectInstance.AddSdkResolvedEnvironmentVariable()` to only
log messages when an SDK attempts to set an environment variable to a
**different value** than what's already set:

- **When values match** (common case): No message is logged, eliminating
the clutter shown in the original issue
- **When values differ** (conflict): A low-importance message is logged
showing both the attempted value and the existing value for diagnostic
purposes
- **Code refactoring**: Extracted duplicate value comparison and logging
logic into a helper method `LogIfValueDiffers` to improve code
maintainability

Updated string resources in `Strings.resx` and all 13 localized `.xlf`
files:
- `SdkEnvironmentVariableAlreadySet`: Now includes both attempted and
existing values in the message
- `SdkEnvironmentVariableAlreadySetBySdk`: Now includes both attempted
and existing values in the message
- Removed `SdkEnvironmentVariableSet` (no longer logging successful
sets)

The underlying functionality is unchanged—environment variables are
still tracked and set correctly, and properties remain visible in the
structured log viewer.

### Testing

- Built MSBuild successfully
- Verified messages no longer appear when values are the same (common
case)
- Messages would appear when values differ (preserves diagnostic value
for conflicts)
- Confirmed existing unit tests pass
- Verified sample projects build successfully
- Confirmed properties are still visible in structured log viewer
(DOTNET_HOST_PATH, etc.)

### Notes

This implementation balances two concerns:
1. **Eliminates noise**: Repeated messages about the same value being
set multiple times no longer clutter logs
2. **Preserves diagnostics**: Actual conflicts (different values) are
still logged with full context (both values shown)

The messages log at `MessageImportance.Low` to provide diagnostic
information without being intrusive in normal builds. Code quality was
improved by extracting duplicate logic into a reusable helper method.

<!-- START COPILOT ORIGINAL PROMPT -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Duplicated messages from SDK/environment
resolution</issue_title>
> <issue_description><img width="2860" height="1772" alt="Image"
src="https://github.com/user-attachments/assets/f4d4871f-28e9-4cb5-9f58-42c4a0056979"
/>
> 
> There are a ton of these `An SDK attempted to set the environment
variable "DOTNET_HOST_PATH"` messages that aren't particularly helpful,
and they're not attributed to a project so they clutter up the top level
of the viewer tool.</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>



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- Fixes #12915

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---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: YuliiaKovalova <[email protected]>

Author: Copilot

src/Build/Instance/ProjectInstance.cs

@@ -1390,13 +1390,13 @@ public void AddSdkResolvedEnvironmentVariable(string name, string value)
             // If the property has already been set as an environment variable, we do not overwrite it.
             if (_environmentVariableProperties.Contains(name))
             {
-                _loggingContext.LogComment(MessageImportance.Low, "SdkEnvironmentVariableAlreadySet", name, value);
+                LogIfValueDiffers(_environmentVariableProperties, name, value, "SdkEnvironmentVariableAlreadySet");
                 return;
             }
             // If another SDK already set it, we do not overwrite it.
             else if (_sdkResolvedEnvironmentVariableProperties?.Contains(name) == true)
             {
-                _loggingContext.LogComment(MessageImportance.Low, "SdkEnvironmentVariableAlreadySetBySdk", name, value);
+                LogIfValueDiffers(_sdkResolvedEnvironmentVariableProperties, name, value, "SdkEnvironmentVariableAlreadySetBySdk");
                 return;
             }
 
@@ -1412,8 +1412,18 @@ public void AddSdkResolvedEnvironmentVariable(string name, string value)
                 ((IEvaluatorData<ProjectPropertyInstance, ProjectItemInstance, ProjectMetadataInstance, ProjectItemDefinitionInstance>)this)
                    .SetProperty(name, value, isGlobalProperty: false, mayBeReserved: false, loggingContext: _loggingContext, isEnvironmentVariable: true, isCommandLineProperty: false);
             }
+        }
 
-            _loggingContext.LogComment(MessageImportance.Low, "SdkEnvironmentVariableSet", name, value);
+        /// <summary>
+        /// Helper method to log a message if the attempted value differs from the existing value.
+        /// </summary>
+        private void LogIfValueDiffers(PropertyDictionary<ProjectPropertyInstance> propertyDictionary, string name, string attemptedValue, string messageResourceName)
+        {
+            ProjectPropertyInstance existingProperty = propertyDictionary.GetProperty(name);
+            if (existingProperty != null && !string.Equals(existingProperty.EvaluatedValue, attemptedValue, StringComparison.Ordinal))
+            {
+                _loggingContext.LogComment(MessageImportance.Low, messageResourceName, name, attemptedValue, existingProperty.EvaluatedValue);
+            }
         }
 
         /// <summary>

src/Build/Resources/Strings.resx

@@ -1384,14 +1384,12 @@ Errors: {3}</value>
     <comment>{StrBegin="MSB4238: "}</comment>
   </data>
   <data name="SdkEnvironmentVariableAlreadySet" xml:space="preserve">
-    <value>An SDK attempted to set the environment variable "{0}" to "{1}" but it was already set as an environment variable.</value>
+    <value>An SDK attempted to set the environment variable "{0}" to "{1}" but it was already set to "{2}" as an environment variable.</value>
   </data>
   <data name="SdkEnvironmentVariableAlreadySetBySdk" xml:space="preserve">
-    <value>An SDK attempted to set the environment variable "{0}" to "{1}" but it was already set by another SDK.</value>
-  </data>
-  <data name="SdkEnvironmentVariableSet" xml:space="preserve">
-    <value>An SDK attempted set the environment variable "{0}" to "{1}".</value>
+    <value>An SDK attempted to set the environment variable "{0}" to "{1}" but it was already set to "{2}" by another SDK.</value>
   </data>
+
   <data name="UnrecognizedParentElement" xml:space="preserve">
     <value>MSB4189: &lt;{1}&gt; is not a valid child of the &lt;{0}&gt; element.</value>
     <comment>{StrBegin="MSB4189: "}</comment>