Require Tool inputSchema during deserialization

[DragonFSKY]

Motivation and Context

Fixes #1575.

Tool.InputSchema defaults to {"type":"object"} for code-created Tool instances. During JSON deserialization, that same default currently masks payloads that omit the required inputSchema property, allowing invalid tool definitions to deserialize successfully.

Changes

• Mark Tool.InputSchema as JSON-required so missing inputSchema fails during deserialization.
• Add a regression test covering deserialization of a tool payload without inputSchema.
• Preserve the existing default schema behavior for code-created Tool instances.

How Has This Been Tested?

• PATH=/home/dragonfsky/.dotnet:$PATH /home/dragonfsky/.dotnet/dotnet build
• PATH=/home/dragonfsky/.dotnet:$PATH /home/dragonfsky/.dotnet/dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj -f net10.0 --filter FullyQualifiedName~ModelContextProtocol.Tests.Protocol.ToolTests
• PATH=/home/dragonfsky/.dotnet:$PATH /home/dragonfsky/.dotnet/dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj -f net10.0 --filter "FullyQualifiedName~ExperimentalPropertySerializationTests|FullyQualifiedName~ExperimentalInternalPropertyTests"
• PATH=/home/dragonfsky/.dotnet:$PATH /home/dragonfsky/.dotnet/dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj -f net10.0 --filter "FullyQualifiedName!~DockerEverythingServerTests"

[halter73]

Thanks for the PR. I agree that aligning with TS and Python (which both reject missing inputSchema) is the right call, and we just cut 2.0 so the timing works for a clean break. Two small bookkeeping requests before merge:

1. Update the PR description's "Breaking Changes" section. It currently says "None"; it should explicitly call out that tools/list responses from servers that omit inputSchema will now throw JsonException during deserialization, where previously they were silently accepted with a null/empty schema.
2. Note in the changelog entry that no backport to the 2.0.x patch series is planned -- this only ships in the next minor.

I've added the breaking-change label.