[release/5.0-preview4] Revert processing bundles in framework (#35679)v5.0.0-preview.4.20251.6

This commit reverts:
Revert "Single-File: Process bundles in the framework (#34274)"
This reverts commit 78b303df8fbb242985d049a277d0d199cafd51b5.

Revert "Single-File Bundler: Add a FileSize test (#35149)"
This reverts commit 779588a509b81d909ac5d496c172949ec1f1ddcc.

*Customer Scenario*

Publishing apps as a self-contained single-file doesn't work as expected.

* Publish needs to generate hostpolicy and hostfxr separate from the single file bundle
* Cross-platform publishing is incorrect

*Problem*

Since Static-apphost is not yet ready, processing bundle content in hostpolicy means that  hostpolicy and hostfxr DLLs need to be separate from the bundle. This causes self-contained single-file apps to not be a "single file" temporarily. 

The change also requires supporting changes from the SDK, to publish hostfxr and hostpolicy as separate files, and to invoke HostModel library with arguments that facilitate cross-platform publishing.

*Solution*

To solve these, problem, this change reverts:

Revert "Single-File: Process bundles in the framework (#34274)" commit 78b303df8fbb242985d049a277d0d199cafd51b5.

and a dependent test-only change:

Revert "Single-File Bundler: Add a FileSize test (#35149)" commit 779588a509b81d909ac5d496c172949ec1f1ddcc.

*Risk*

Medium
The change is contained to only host components: apphost, hostpolicy, and hostfxr.
However, the change is big, and needs testing in runtime and SDK repos.

*Testing*

Manually tested the SDK by inserting apphost, hostfxr, hostpolicy, and hostmodel library from this build into the `dotnet/packs` preview-4 SDK from https://github.com/dotnet/sdk/pull/11518 build.

Verified that:
* Singlefile apps can be published and run OK for  { Windows, Linux, Osx } x {netcoreapp3.0, netcoreapp3.1, netcoreapp5.0} 
* Cross-targeting builds of single-file apps build and run OK (ex: built on Windos, run on Mac).

1 files changed, 72 insertions, 0 deletions

diff --git a/src/installer/corehost/cli/apphost/bundle/file_entry.h b/src/installer/corehost/cli/apphost/bundle/file_entry.h
new file mode 100644
index 00000000000..efe405cc9a6
--- /dev/null
+++ b/src/installer/corehost/cli/apphost/bundle/file_entry.h
@@ -0,0 +1,72 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#ifndef __FILE_ENTRY_H__
+#define __FILE_ENTRY_H__
+
+#include "file_type.h"
+#include "reader.h"
+
+namespace bundle
+{
+    // FileEntry: Records information about embedded files.
+    // 
+    // The bundle manifest records the following meta-data for each 
+    // file embedded in the bundle:
+    // Fixed size portion (file_entry_fixed_t)
+    //   - Offset     
+    //   - Size       
+    //   - File Entry Type       
+    // Variable Size portion
+    //   - relative path (7-bit extension encoded length prefixed string)
+
+#pragma pack(push, 1)
+    struct file_entry_fixed_t
+    {
+        int64_t offset;
+        int64_t size;
+        file_type_t type;
+    };
+#pragma pack(pop)
+
+    class file_entry_t
+    {
+    public:
+        file_entry_t()
+            : m_offset(0)
+            , m_size(0)
+            , m_type(file_type_t::__last)
+            , m_relative_path()
+        {
+        }
+
+        file_entry_t(const file_entry_fixed_t *fixed_data)
+            :m_relative_path()
+        {
+            // File_entries in the bundle-manifest are expected to be used 
+            // beyond startup (for loading files directly from bundle, lazy extraction, etc.).
+            // The contents of fixed_data are copied on to file_entry in order to 
+            // avoid memory mapped IO later.
+
+            m_offset = fixed_data->offset;
+            m_size = fixed_data->size;
+            m_type = fixed_data->type;
+        }
+
+        const pal::string_t relative_path() const { return m_relative_path; }
+        int64_t offset() const { return m_offset; }
+        int64_t size() const { return m_size; }
+        file_type_t type() const { return m_type; }
+
+        static file_entry_t read(reader_t &reader);
+
+    private:
+        int64_t m_offset;
+        int64_t m_size;
+        file_type_t m_type;
+        pal::string_t m_relative_path; // Path of an embedded file, relative to the extraction directory.
+        bool is_valid() const;
+    };
+}
+#endif // __FILE_ENTRY_H__