Add -f/--framework support to TUnit template with .NET Framework handling

[Copilot]

The TUnit template lacked -f/--framework support, making it impossible to scaffold projects targeting specific frameworks — particularly .NET Framework, which requires additional project configuration (polyfills, warning suppression, lang version).

Changes

• template.json — Added framework choice parameter (net8.0–net10.0, net462–net481) with computed IsNetFramework symbol and conditional source exclusion for the Polyfills/ directory
• TestProject.csproj — Conditional .NET Framework block: LangVersion=preview, EnableTUnitPolyfills=false, NoWarn for CS0592/CS0436, and Polyfill package reference
• Polyfills/ExcludeFromCodeCoverageAttribute.cs — Compatibility shim only included for .NET Framework targets
• Template tests — Added InstantiationTestWithNetFramework and OptionsWithFramework() helper; accepted snapshot

Usage

dotnet new TUnit -n MyTests -f net8.0    # modern .NET (clean project)
dotnet new TUnit -n MyTests -f net48     # .NET Framework (polyfills + warnings suppressed)
dotnet new TUnit -n MyTests              # defaults to net10.0

[codacy-production]

Not up to standards ⛔

🔴 Issues 1 medium

Alerts:
⚠ 1 issue (≤ 0 issues of at least minor severity)

Results:
1 new issue

Category	Results
BestPractice	1 medium

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🟢 Metrics 21 complexity

Metric	Results
Complexity	21

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