feat(ir): add typed builder for UseDotNet@2

[github-actions]

Summary

Adds a typed builder struct for UseDotNet@2 to the ado-aw IR, and migrates the stringly-typed dotnet_install_task_step factory in src/runtimes/dotnet/extension.rs.

Motivation

Previously, the .NET runtime extension emitted the UseDotNet@2 step via raw TaskStep::new("UseDotNet@2", ...).with_input("packageType", "sdk").with_input(...) calls. This PR introduces a typed builder so required inputs are positional, optional inputs are type-checked, and the PackageType constraint ("sdk" / "runtime") is modelled as an enum rather than a stringly-typed key.

Changes

• src/compile/ir/tasks/use_dotnet.rs: new UseDotNet builder struct with PackageType enum, convenience constructors (with_version / with_global_json), typed optional setters, and #[cfg(test)] mod tests (11 unit tests)
• src/compile/ir/tasks/mod.rs: pub mod use_dotnet; declaration (alphabetical order)
• src/runtimes/dotnet/extension.rs: migrated dotnet_install_task_step from raw TaskStep::new to UseDotNet::with_global_json() / UseDotNet::with_version(), added use_dotnet::UseDotNet import
• src/compile/extensions/tests.rs: updated two assertions that expected the now-omitted explicit packageType: sdk (the ADO default is "sdk", so the typed builder correctly omits it to keep generated YAML minimal)

ADO Task Reference

• Task: UseDotNet@2
• Docs: https://learn.microsoft.com/en-us/azure/devops/pipelines/tasks/reference/use-dotnet-v2-task
• Required inputs: (none — all inputs are optional)
• Optional inputs: version, packageType (PackageType enum), useGlobalJson, workingDirectory, installationPath, performMultiLevelLookup, failOnStandardError

Builder modes

// Version spec mode
UseDotNet::with_version("8.0.x").into_step()

// global.json mode
UseDotNet::with_global_json()
    .with_display_name("Install .NET SDK (from global.json)")
    .into_step()

// Runtime (not SDK)
UseDotNet::with_version("8.0.x")
    .package_type(PackageType::Runtime)
    .into_step()

Validation

• cargo build --all-targets
• cargo test (2133 tests pass, 0 failed)
• cargo clippy --all-targets --all-features --workspace -- -D warnings

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