Fix memory leaks from mono_type_full_name in marshal-shared.c error paths (dotnet#125616)

`mono_type_full_name()` returns a heap-allocated string, but several
call sites in `marshal-shared.c` passed the result directly into
`g_strdup_printf` without freeing it — leaking on every hit. These paths
can fire repeatedly during AOT compilation against assemblies with many
invalid marshal configurations.

# Description

Store each `mono_type_full_name()` result in a local, use it, then
`g_free` it:

```c
// Before
char *msg = g_strdup_printf("Type %s ...", mono_type_full_name(m_class_get_byval_arg(klass)));

// After
char *type_name = mono_type_full_name(m_class_get_byval_arg(klass));
char *msg = g_strdup_printf("Type %s ...", type_name);
g_free(type_name);
```

Fixed sites:
- **Auto-layout check** — `g_strdup_printf` with single
`mono_type_full_name`; result stored, used, and freed
- **Explicit-layout `g_error` path** — left as-is (inline
`mono_type_full_name` call); `g_error` is fatal and never returns, and
in `DISABLE_ASSERT_MESSAGES` builds its variadic args are discarded
entirely — introducing a local variable causes an unused-variable build
error in WASM/WASI configurations
- **Generic type marshal check** — `g_strdup_printf` with single
`mono_type_full_name`; result stored, used, and freed
- **Struct field type check** — `g_strdup_printf` with two
`mono_type_full_name` calls; both results stored, used, and freed

# Customer Impact

Cumulative memory growth during AOT compilation of assemblies containing
types with invalid marshal configurations (auto-layout structs,
non-blittable generic fields, reference-typed struct fields).

# Regression

No — pre-existing leak, not a regression.

# Testing

Validated by code inspection and CI. These are error paths with no
dedicated test infrastructure in the repo; the fix is mechanical and
low-risk.

# Risk

Very low. Changes are confined to error paths that emit a marshal
exception. No logic changes — only introduction of local variables and
paired `g_free` calls before strings go out of scope. The `g_error` path
is intentionally left unchanged to avoid a `-Werror,-Wunused-variable`
build failure in WASM/WASI configurations where `g_error` discards its
variadic arguments.

# Package authoring no longer needed in .NET 9

IMPORTANT: Starting with .NET 9, you no longer need to edit a NuGet
package's csproj to enable building and bump the version.
Keep in mind that we still need package authoring in .NET 8 and older
versions.

<!-- START COPILOT ORIGINAL PROMPT -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Memory leaks from mono_type_full_name in marshal-shared.c
error paths</issue_title>
> <issue_description>## Description
> 
> `mono_type_full_name()` returns an allocated string, but several call
sites in `src/mono/mono/metadata/marshal-shared.c` pass it directly into
`g_strdup_printf` or `g_error` without freeing the intermediate
allocation:
> 
> - **Line 778**: `g_strdup_printf("Type %s ...",
mono_type_full_name(...))`
> - **Line 816**: `g_error("Type %s ...", mono_type_full_name(...))`
> - **Line 866**: `g_strdup_printf("Generic type %s ...",
mono_type_full_name(...))`
> - **Lines 912-913**: `g_strdup_printf("Type %s with field type %s
...", mono_type_full_name(...), mono_type_full_name(...))`
> 
> Each leaks the string returned by `mono_type_full_name()`. While these
are error paths, they can fire repeatedly during AOT compilation (e.g.,
when processing assemblies with many invalid marshal configurations),
causing cumulative memory growth.
> 
> ## Fix
> 
> Store the result in a temporary, use it in the format string, then
`g_free` it:
> 
> ```c
> char *type_name = mono_type_full_name(m_class_get_byval_arg(klass));
> char *msg = g_strdup_printf("Type %s ...", type_name);
> g_free(type_name);
> mono_marshal_shared_mb_emit_exception_marshal_directive(mb, msg);
> ```
> 
> This pattern should be applied to all call sites in the
file.</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>



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- Fixes dotnet#125576

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: lewing <24063+lewing@users.noreply.github.com>
Co-authored-by: Larry Ewing <lewing@microsoft.com>

Author: Copilot

src/mono/mono/metadata/marshal-shared.c

@@ -774,8 +774,10 @@ mono_marshal_shared_emit_struct_conv_full (MonoMethodBuilder *mb, MonoClass *kla
 
 	if (klass != mono_class_try_get_safehandle_class ()) {
 		if (mono_class_is_auto_layout (klass)) {
+			char *type_name = mono_type_full_name (m_class_get_byval_arg (klass));
 			char *msg = g_strdup_printf ("Type %s which is passed to unmanaged code must have a StructLayout attribute.",
-										 mono_type_full_name (m_class_get_byval_arg (klass)));
+										 type_name);
+			g_free (type_name);
 			mono_marshal_shared_mb_emit_exception_marshal_directive (mb, msg);
 			return;
 		}
@@ -862,8 +864,10 @@ mono_marshal_shared_emit_struct_conv_full (MonoMethodBuilder *mb, MonoClass *kla
 				break;
 			case MONO_TYPE_GENERICINST:
 				if (!mono_type_generic_inst_is_valuetype (ftype)) {
+					char *type_name = mono_type_full_name (ftype);
 					char *msg = g_strdup_printf ("Generic type %s cannot be marshaled as field in a struct.",
-						mono_type_full_name (ftype));
+						type_name);
+					g_free (type_name);
 					mono_marshal_shared_mb_emit_exception_marshal_directive (mb, msg);
 					break;
 				}
@@ -908,9 +912,13 @@ mono_marshal_shared_emit_struct_conv_full (MonoMethodBuilder *mb, MonoClass *kla
 			case MONO_TYPE_CLASS:
 			case MONO_TYPE_SZARRAY:
 			case MONO_TYPE_ARRAY: {
+				char *klass_name = mono_type_full_name (m_class_get_byval_arg (klass));
+				char *field_type_name = mono_type_full_name (ftype);
 				char *msg = g_strdup_printf ("Type %s with field type %s cannot be marshaled as an unmanaged struct field.",
-					mono_type_full_name (m_class_get_byval_arg (klass)),
-					mono_type_full_name (ftype));
+					klass_name,
+					field_type_name);
+				g_free (klass_name);
+				g_free (field_type_name);
 				mono_marshal_shared_mb_emit_exception_marshal_directive (mb, msg);
 				break;
 			}