Fix initial position parsing from -pos tag (#19409)

## Summary of the Pull Request
Fixes #4247. Improves consistency in -pos tag parsing by supporting
single-value input for both axes.

## References and Relevant Issues
#4247

## Detailed Description of the Pull Request / Additional comments
The change aligns -pos behavior with padding-style logic. Existing
comma-separated behavior remains unchanged.
- A single value like "number" now sets both x and y positions to that
value — similar to how padding behaves.
- A value like "number," (with a trailing comma) continues to set only
x, leaving y unchanged.

## Validation Steps Performed
- Ran all unit and integration tests
- Added unit test to verify single-value and comma-separated inputs

## PR Checklist
- [x] Closes #4247
- [x] Tests added/passed
- [x] Documentation updated
https://github.com/MicrosoftDocs/terminal/pull/890

---------

Co-authored-by: Carlos Zamora <cazamor@microsoft.com>

doc/specs/#1043 - Set the initial position of the Terminal/spec.md

@@ -27,6 +27,7 @@ Two new properties should be added in the json settings file:
 
 2. Both X value and Y values are optional. If any one of them is missing, or the value is invalid, system default value will be used. Examples:
 
+       "1000" equals (1000, 1000)
        ", 1000" equals (default, 1000)
        "1000, " equals (1000, default)
        "," equals (default, default)

src/cascadia/TerminalSettingsModel/ModelSerializationHelpers.h

@@ -16,10 +16,10 @@ Abstract:
 // - Helper for converting a pair of comma separated, potentially absent integer values
 //   into the corresponding left and right values. The leftValue and rightValue functions
 //   will be called back with the associated parsed integer value, assuming it's present.
-//   (100, 100): leftValue and rightValue functions both called back with 100.
+//   (100, 100), (100): leftValue and rightValue functions both called back with 100.
 //   (100, ), (100, abc): leftValue function called back with 100
 //   (, 100), (abc, 100): rightValue function called back with 100
-//   (,): no function called back
+//   (,), (abc): no function called back
 //   (100, 100, 100): we only read the first two values, this is equivalent to (100, 100)
 // Arguments:
 // - string: The string to parse
@@ -45,6 +45,10 @@ _TIL_INLINEPREFIX void ParseCommaSeparatedPair(const std::string& string,
             if (index == 0)
             {
                 leftValue(value);
+                if (string.find(',') == std::string::npos)
+                {
+                    rightValue(value);
+                }
             }
 
             if (index == 1)

src/cascadia/UnitTests_SettingsModel/TerminalSettingsTests.cpp

@@ -7,6 +7,7 @@
 #include <pcg_random.hpp>
 
 #include "../TerminalSettingsModel/CascadiaSettings.h"
+#include "../TerminalSettingsModel/ModelSerializationHelpers.h"
 #include "TestUtils.h"
 
 using namespace Microsoft::Console;
@@ -52,6 +53,7 @@ namespace SettingsModelUnitTests
         TEST_METHOD(MakeSettingsForDefaultProfileThatDoesntExist);
         TEST_METHOD(TestLayerProfileOnColorScheme);
         TEST_METHOD(TestCommandlineToTitlePromotion);
+        TEST_METHOD(TestInitialPositionParsing);
     };
 
     // CascadiaSettings::_normalizeCommandLine abuses some aspects from CommandLineToArgvW
@@ -891,4 +893,49 @@ namespace SettingsModelUnitTests
             VERIFY_ARE_EQUAL(L"", settingsStruct.DefaultSettings()->StartingTitle());
         }
     }
+    void TerminalSettingsTests::TestInitialPositionParsing()
+    {
+        {
+            const auto pos = LaunchPositionFromString("50");
+            VERIFY_IS_TRUE(pos.X.Value());
+            VERIFY_IS_TRUE(pos.Y.Value());
+            VERIFY_ARE_EQUAL(50, static_cast<int32_t>(pos.X.Value()));
+            VERIFY_ARE_EQUAL(50, static_cast<int32_t>(pos.Y.Value()));
+        }
+
+        {
+            const auto pos = LaunchPositionFromString("100,");
+            VERIFY_IS_TRUE(pos.X.Value());
+            VERIFY_ARE_EQUAL(100, static_cast<int32_t>(pos.X.Value()));
+            VERIFY_IS_TRUE(pos.Y == nullptr);
+        }
+
+        {
+            const auto pos = LaunchPositionFromString(",100");
+            VERIFY_IS_TRUE(pos.X == nullptr);
+            VERIFY_IS_TRUE(pos.Y.Value());
+            VERIFY_ARE_EQUAL(100, static_cast<int32_t>(pos.Y.Value()));
+        }
+
+        {
+            const auto pos = LaunchPositionFromString("50,50");
+            VERIFY_IS_TRUE(pos.X.Value());
+            VERIFY_ARE_EQUAL(50, static_cast<int32_t>(pos.X.Value()));
+            VERIFY_IS_TRUE(pos.Y.Value());
+            VERIFY_ARE_EQUAL(50, static_cast<int32_t>(pos.Y.Value()));
+        }
+
+        {
+            const auto pos = LaunchPositionFromString("abc,100");
+            VERIFY_IS_TRUE(pos.X == nullptr);
+            VERIFY_IS_TRUE(pos.Y.Value());
+            VERIFY_ARE_EQUAL(100, static_cast<int32_t>(pos.Y.Value()));
+        }
+
+        {
+            const auto pos = LaunchPositionFromString("abc");
+            VERIFY_IS_TRUE(pos.X == nullptr);
+            VERIFY_IS_TRUE(pos.Y == nullptr);
+        }
+    }
 }