[MSBUILD SDK] Add support for multi-targeted function apps

[jviau]

Issue describing the changes in this PR

resolves #3331

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• I have added all required tests (Unit tests, E2E tests)

Additional information

This PR adds support for multi-targeted function apps. When a multi-targeted app is detected, an extension project will be generated and restored per TFM.

Important

Microsoft.NET.Sdk/10.0.200 includes NuGet 7.3, which introduces a new TFM alias concept. This concept may allow us to switch back to a single project, but multiple TFM aliases to create separate restore graphs. TFM Alias will not be fully supported until 10.0.300 though.

[Copilot]

Pull request overview

Adds MSBuild SDK support for multi-targeted Azure Functions apps by generating/restoring a Functions extensions project per TFM.

Changes:

• Update extension generation pipeline to operate on per-TFM project items (projects + packages tagged with TFMs).
• Extend validation/resolution tasks and targets to carry TFM metadata through restore/build.
• Expand unit + integration tests to cover multi-target restore/build and incremental behavior.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
test/Azure.Functions.Sdk.Tests/Tasks/Extensions/WriteExtensionProjectTests.cs	Updates tests for new per-TFM project/hash outputs and adds multi-target coverage.
test/Azure.Functions.Sdk.Tests/Tasks/Extensions/ValidateExtensionPackagesTests.cs	Updates package creation to include TFMs; adds warnings/behavior tests for missing/different TFMs.
test/Azure.Functions.Sdk.Tests/Tasks/Extensions/ResolveExtensionPackagesTests.cs	Ensures resolved extension packages carry TFM metadata; adds multi-target scan test.
test/Azure.Functions.Sdk.Tests/ProjectCreatorExtensions.cs	Adds helper APIs for multi-target project creation and per-TFM extension project directory resolution.
test/Azure.Functions.Sdk.Tests/Integration/SdkEndToEndTests.Restore.cs	Expands restore E2E tests across TFMs and multi-target scenarios; centralizes generated project path logic.
test/Azure.Functions.Sdk.Tests/Integration/SdkEndToEndTests.Build.cs	Expands build E2E tests across TFMs and multi-target scenarios, including incremental checks.
src/Azure.Functions.Sdk/Tasks/Extensions/WriteExtensionProject.cs	Refactors task to write one extension project per TFM and include TFM in hashing.
src/Azure.Functions.Sdk/Tasks/Extensions/ValidateExtensionPackages.cs	Validates packages per TFM; warns and filters out packages missing TFM metadata.
src/Azure.Functions.Sdk/Tasks/Extensions/ResolveExtensionPackages.cs	Tags resolved extension packages with TFM metadata.
src/Azure.Functions.Sdk/TaskItemExtensions.cs	Adds strongly-typed accessor for TargetFramework item metadata.
src/Azure.Functions.Sdk/Targets/Extensions/Azure.Functions.Sdk.Extensions.targets	Updates targets to generate/restore per-TFM extension projects and markers; backfills assets path for multi-target.
src/Azure.Functions.Sdk/Targets/Extensions/Azure.Functions.Sdk.Extensions.props	Removes single-project path properties now replaced by per-TFM item metadata.
src/Azure.Functions.Sdk/Strings.resx	Adds warning string for missing extension package TFM metadata.
src/Azure.Functions.Sdk/LogMessage.cs	Registers new warning log message identifier.

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