Enable EnablePackageValidation for all src packages with 0.9.0-preview.1 baseline

[Copilot]

Opts into EnablePackageValidation so the build catches API breaking changes automatically against the published 0.9.0-preview.1 baseline.

Changes

• src/Directory.Build.props: Adds EnablePackageValidation=true and PackageValidationBaselineVersion=0.9.0-preview.1 — applies to all three packable projects (ModelContextProtocol.Core, ModelContextProtocol, ModelContextProtocol.AspNetCore)
• src/ModelContextProtocol.Core/CompatibilitySuppressions.xml: Suppresses intentional breaking changes already made post-0.9.0-preview.1 (property type changes on ProtectedResourceMetadata, DynamicClientRegistrationResponse, ToolResultContentBlock.Content, and ListTasksResult.Tasks). All suppressions are marked IsBaselineSuppression=true.

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

Original prompt

---

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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[eiriktsarpalis]

Is this adding suppressions for experimental APIs presumably?

[eiriktsarpalis]

Maybe we can keep this turned off until we have a final stable release.

Suggested change

	<EnablePackageValidation>true</EnablePackageValidation>
	<EnablePackageValidation>false</EnablePackageValidation>

[stephentoub]

Why do we need any suppressions right now?

[jeffhandley]

Because we've already made changes in main; and this caught them. But I'll wait to finalize this PR until we cut 1.0.0 stable, and clear the suppressions.