Enable EnablePackageValidation for all src packages with 1.0.0 baseline

[Copilot]

Opts into EnablePackageValidation so the build catches API breaking changes automatically against the published 1.0.0 stable baseline.

Changes

• src/Directory.Build.props: Adds EnablePackageValidation=true and PackageValidationBaselineVersion=1.0.0 — applies to all three packable projects (ModelContextProtocol.Core, ModelContextProtocol, ModelContextProtocol.AspNetCore)

Any new breaking change introduced going forward will fail dotnet pack unless explicitly suppressed.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Opt in to EnablePackageValidation to catch breaking changes</issue_title>
<issue_description>Skills aside, we should enable the EnablePackageValidation property in the main projects so that breaking changes are caught by the build process itself. That should provide much stronger feedback to LLMs as well.

Originally posted by @eiriktsarpalis in #1328 (comment)
</issue_description>

<agent_instructions>The 0.9.0-preview.1 version should be used as the baseline to establish for package validation. Validation should be enabled for all packages produced in the src build.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Opt in to EnablePackageValidation to catch breaking changes #1329

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[stephentoub]

@copilot since this PR was created, the libraries have moved to a stable 1.0.0 release. Please update this PR accordingly, including resetting the baselining which should no longer be necessary.

[Copilot]

@copilot since this PR was created, the libraries have moved to a stable 1.0.0 release. Please update this PR accordingly, including resetting the baselining which should no longer be ...

Updated in 1c81385 — baseline is now 1.0.0 and the CompatibilitySuppressions.xml has been removed since no suppressions are needed against the stable release.

[stephentoub]

@copilot can you temporarily make a breaking API change and validate that the build fails / catches it?

[Copilot]

@copilot can you temporarily make a breaking API change and validate that the build fails / catches it?

Verified — I temporarily changed Annotations.Priority from public to internal and ran dotnet pack. The build compiled fine but package validation caught it with CP0002 errors across all four TFMs:

error CP0002: Member 'float? ModelContextProtocol.Protocol.Annotations.Priority.get' exists on [Baseline] lib/net8.0/ModelContextProtocol.Core.dll but not on lib/net8.0/ModelContextProtocol.Core.dll
error CP0002: Member 'void ModelContextProtocol.Protocol.Annotations.Priority.set' exists on [Baseline] lib/net8.0/ModelContextProtocol.Core.dll but not on lib/net8.0/ModelContextProtocol.Core.dll

(repeated for net9.0, net10.0, and netstandard2.0)

Ending with: error : API breaking changes found. If those are intentional, the APICompat suppression file can be updated by rebuilding with '/p:ApiCompatGenerateSuppressionFile=true'

The temporary change has been reverted — no code changes needed.