Fix syntax highlighting for tuple deconstruction in for loop initializers

[Copilot]

Tuple deconstruction in for loop initializers was not being tokenized correctly, breaking syntax highlighting for the entire statement and subsequent code.

// Previously broken highlighting
for ((int i, int a1, int a2) = (0, 0, 0); i < 10; i++) { }

// Also affected var syntax
for (var (i, a1, a2) = (0, 0, 0); i < 10; i++) { }

Changes

Grammar (src/csharp.tmLanguage.yml)

• Added #local-tuple-var-deconstruction to for-statement initializer patterns
• Added #tuple-deconstruction-assignment to recognize explicit type tuple patterns
• Added #expression to parse assignment right-hand side

Tests (test/statements.tests.ts)

• Added test coverage for explicit type tuple deconstruction in for loops
• Added test coverage for var tuple deconstruction in for loops

Original prompt

---

This section details on the original issue you should resolve

<issue_title>C# for loop with tuple initializer breaks color highlighting</issue_title>
<issue_description>## Details

What editor are you seeing the problem in?
Visual Studio Code

What version of the editor are you using?
1.101.1

What color theme are you using?
Dark+
AposTheme

Repro

for ((int i, int a1, int a2) = (0, 0, 0); i < 10; i++) { }

(int b1, int b2, int b3) = (0, 0, 0);

Dark+:

AposTheme:
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes C# for loop with tuple initializer breaks color highlighting #331

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