[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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[chrikrah]

@Coding-Professional, this covers rules 1 to 4 of #1868 with tests, and caps resume. From my perspective it can be merged; the API call stays open on #1868.

non-blocking: both ceiling .Where filters can't fire before a revision after 2026-07-28, so removing them still passes 58. I'd keep them. Could you add a comment that the next revision needs a ceiling test?

$ dotnet test tests/ModelContextProtocol.Tests -f net10.0 --filter "FullyQualifiedName~July2026ProtocolFallbackTests"   # head dd3bef4, git archive export, sdk 10.0.403
Passed: 58
$ <same, both ceiling filters removed from McpClientImpl.cs>
Passed: 58

[Coding-Professional]

CI after rebasing onto upstream main (64cfd66b): five of six build/test matrix jobs passed. Ubuntu Release built successfully and passed its .NET 10 and .NET 9 core test runs. Its .NET 8 core test host was then terminated by the configured seven-minute inactivity timeout, with 3,674 tests passed and five skipped before the abort. The blame log lists StdioClientTransportTests.InheritEnvironmentVariables_False_ChildDoesNotSeeParentEnvVars as the active test; this PR does not change that test or the stdio transport.

The focused protocol negotiation tests pass locally (58/58 on net10.0) against the rebased branch. I cannot rerun the upstream job from my contributor account. Could a maintainer please rerun the failed Ubuntu Release job?