Fix CI build: preserve binary compat for FindMappedElementByChoiceName

The Azure Pipelines build failed on the Pack step with API compatibility
error CP0002: adding an optional 'ignoreCase' parameter to the existing
public FindMappedElementByChoiceName(string) method changed its IL
signature, breaking binary compatibility against the 6.2.0 baseline
(EnablePackageValidation is on for this repo).

Split into two overloads instead: the original single-parameter method
is preserved unchanged and delegates to a new two-parameter overload.
Verified locally with 'dotnet pack --no-build', which reproduces the
same API compat check the pipeline runs.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: ewoutkramer

src/Hl7.Fhir.Base/Introspection/ClassMapping.cs

@@ -243,7 +243,19 @@ PropertyMappingCollection createCollection()
     /// so for where there is no suffix. In this case, however, <see cref="FindMappedElementByName(string)"/>
     /// is faster.
     /// </remarks>
-    public PropertyMapping? FindMappedElementByChoiceName(string name, bool ignoreCase = false)
+    public PropertyMapping? FindMappedElementByChoiceName(string name) => FindMappedElementByChoiceName(name, ignoreCase: false);
+
+    /// <summary>
+    /// Returns the mapping for an element of this class by a name that
+    /// might be suffixed by a type name (e.g. for choice elements).
+    /// </summary>
+    /// <param name="name">The (possibly suffixed) name to look up.</param>
+    /// <param name="ignoreCase">If <c>true</c>, the choice prefix is matched case-insensitively.</param>
+    /// <remarks>Will also return properties for which the name is exactly the same,
+    /// so for where there is no suffix. In this case, however, <see cref="FindMappedElementByName(string)"/>
+    /// is faster.
+    /// </remarks>
+    public PropertyMapping? FindMappedElementByChoiceName(string name, bool ignoreCase)
     {
         if (name == null) throw Error.ArgumentNull(nameof(name));