[Coding-Professional]

Summary

Add McpClientOptions.MaximumProtocolVersion so clients can choose an upper protocol bound while accepting an older mutually supported revision. This is useful for MCP hosts and gateways that have validated a limited range of protocol revisions.

Fixes #1868.

Behavior

• Validate the ceiling against SDK-supported revisions and reject configuration that also sets the existing exact ProtocolVersion pin before opening a transport.
• A ceiling on an initialize-handshake revision requests that revision directly and accepts an older supported response. A 2026-07-28 ceiling retains discovery and fallback, filtering advertised versions before selecting one.
• Reject malformed/newer initialize responses and advertised version sets with no compatible revision. Resumed sessions cannot exceed the configured ceiling.
• Leave connection behavior unchanged when the new option is unset. Document the difference between an exact pin and a ceiling.

Verification

• Focused protocol negotiation tests: 58 passed on Windows net10.0 and net8.0. These cover ceiling match, downgrade, discovery fallback, no overlap, invalid response, conflicting options, and resume.
• dotnet build src/ModelContextProtocol.Core/ModelContextProtocol.Core.csproj --no-restore: passed for netstandard2.0, net8.0, net9.0, and net10.0 with no warnings or errors.
• git diff --check: passed.
• dotnet format whitespace --verify-no-changes --include ... reports existing whitespace violations on untouched lines in McpClient.Methods.cs and McpClientImpl.cs; this PR does not reformat those unrelated sections.

AI assistance was used to investigate and draft this change; I reviewed the implementation and ran the tests above.

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