[MSBUILD SDK] Add warning for building generated project in Azure.Functions.Sdk

[jviau]

Issue describing the changes in this PR

resolves #issue_for_this_pr

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

1. Makes building the inner project a no-op. Adds a warning in the project is ever built.
2. Changes extension package validation from a warning to an error. This shouldn't ever happen in a normal flow, but this is a safeguard to make sure the package collection hasn't been improperly tampered with.

[Copilot]

Pull request overview

This PR makes two changes to the Azure Functions SDK build tooling: (1) it adds a Build target override in the inner generated project (azure_functions.g.csproj) that emits a warning if the project is accidentally built directly, and (2) it promotes the extension package target framework validation from a warning to an error. New documentation for rules AZFW0109 and AZFW0110 is included, along with corresponding test updates.

Changes:

• Override the Build target in the inner generated project to emit warning AZFW0110 instead of performing a real build.
• Escalate the missing-target-framework validation (AZFW0109) from warning to error, returning false from the task.
• Add documentation for AZFW0109 and AZFW0110, plus integration/unit test updates.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/Azure.Functions.Sdk/Targets/Inner/Azure.Functions.Sdk.Inner.targets	Adds FuncSdkLog UsingTask and overrides Build target to emit a warning
src/Azure.Functions.Sdk/Tasks/Extensions/ValidateExtensionPackages.cs	Changes TFM-missing log from warning to error and returns false
src/Azure.Functions.Sdk/LogMessage.cs	Renames warning to error for AZFW0109, adds new AZFW0110 warning message
src/Azure.Functions.Sdk/Strings.resx	Renames AZFW0109 resource, adds AZFW0110 resource string
docs/sdk-rules/AZFW0108.md	Removes extra blank line
docs/sdk-rules/AZFW0109.md	New documentation for AZFW0109 error rule
docs/sdk-rules/AZFW0110.md	New documentation for AZFW0110 warning rule
test/.../ValidateExtensionPackagesTests.cs	Updates test to expect error instead of warning
test/.../SdkEndToEndTests.Targets.InnerBuild.cs	New integration test for inner build warning

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