Improve diagnostics.source.

[sean-mcmanus]

This is primarily for if/when microsoft/vscode#319418 gets fixed so Copilot can more easily know it's a not an actual C/C++ compiler error.

...but I figured it might help new users as well.

[Copilot]

Pull request overview

Refines the diagnostic.source strings used for diagnostics emitted by the C/C++ extension so consumers (e.g. Copilot) can distinguish IntelliSense, clang-tidy, and refactor diagnostics from generic "C/C++" compiler diagnostics. Introduces a new CppIntelliSenseSourceStr constant and updates the missing-include detection to match the new source value.

Changes:

• Add CppIntelliSenseSourceStr ("C/C++ IntelliSense") and update isMissingIncludeDiagnostic to compare against it.
• Tag clang-tidy and refactor diagnostics with more specific source strings ("C/C++ clang-tidy", "C/C++ Refactor").
• Set IntelliSense diagnostics' source to CppIntelliSenseSourceStr in DefaultClient.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
Extension/src/LanguageServer/extension.ts	Adds the CppIntelliSenseSourceStr constant and updates the missing-include diagnostic check to use it.
Extension/src/LanguageServer/codeAnalysis.ts	Tags clang-tidy diagnostics with the more specific "C/C++ clang-tidy" source.
Extension/src/LanguageServer/client.ts	Imports the new constant, tags refactor diagnostics with "C/C++ Refactor", and sets IntelliSense diagnostics' source to CppIntelliSenseSourceStr.

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[sean-mcmanus]

I want to make sure @bobbrow is okay with this before checking it in.