Add diagnostic for missing MessagePack submodule sources

[PureWeen]

Add diagnostic for missing MessagePack submodule sources

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Add a focused diagnostic for missing MessagePack submodule sources

Description

While using Copilot to reproduce an unrelated Components issue in a fresh linked worktree, incomplete local setup ended up dominating the investigation. The MessagePack-CSharp submodule was not initialized, but Components.Server reported that prerequisite as a cascade of missing-source CS2001 compiler errors with no indication of how to fix the checkout.

This keeps the diagnostic close to the project that consumes the sources. The 15 MessagePack files are defined once, used as the Compile input, and validated from the same list before CoreCompile. If any consumed source is missing, the build stops with one focused message explaining that an uninitialized submodule may be the cause and provides:

git submodule update --init src/submodules/MessagePack-CSharp

This helps contributors and coding agents diagnose incomplete local setup quickly. It does not mutate the checkout or hide genuinely missing files.

Validation:

• A fresh worktree with the submodule uninitialized produced repeated MessagePack CS2001 errors before this change; afterward it produces one actionable build error and zero CS2001 errors.
• Temporarily removing a consumed non-BufferWriter file verifies that validation covers the authoritative source list rather than a single sentinel.
• Initializing or restoring the sources allows the targeted Components.Server build to succeed.

[Copilot]

Pull request overview

Adds an early, actionable MSBuild diagnostic in Components Server to detect missing MessagePack-CSharp submodule source files and fail the build before the compiler emits a cascade of CS2001 missing-source errors.

Changes:

• Refactors the MessagePack Compile includes into a single authoritative item list (@(_MessagePackSource)).
• Adds a ValidateMessagePackSources target that checks for missing source files before CoreCompile and emits a focused error with the recommended git submodule update --init ... command.

Show a summary per file

File	Description
src/Components/Server/src/Microsoft.AspNetCore.Components.Server.csproj	Centralizes MessagePack source includes and adds a pre-compile validation target to emit a single actionable diagnostic when the submodule sources are missing.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 1