Split long-running Dependabot NuGet groups

[Copilot]

The September 28 update batch had four C# NuGet groups still discovering or restoring projects when their runs were canceled after roughly 55 minutes. Split those groups further while preserving their existing update settings and directory coverage.

• Groups split: System/A*–E* into A–C and D–E; System.A*/System.B*/System.CodeDom*/System.Collections* into two groups; System.D* into Data/Diagnostics and Drawing/Dynamic; and System.S*/System.T* into separate groups.
• Example — System.D*:

  directories:
    - "snippets/csharp/System.Data*/**/*"
    - "snippets/csharp/System.Diagnostics*/**/*"

[Copilot]

Copilot review overview

🟢 Approval recommended

The split preserves existing project coverage and update settings without introducing overlap or omissions.

Review effort: Balanced
Findings: None

What changed in this PR

Splits long-running C# NuGet Dependabot groups while preserving project coverage and update settings.

Changes:

• Divides four large directory groups into smaller update jobs.
• Retains weekly schedules, cooldowns, grouping, and PR limits.

File	Description
.github/​dependabot.yml	Splits C# NuGet directory groups for faster Dependabot runs.

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