fix(client): allow missing server info in discovery

[Coding-Professional]

Problem and change

Under the 2026-07-28 protocol, a server may omit io.modelcontextprotocol/serverInfo from its server/discover response. Today, reading McpClient.ServerInfo after that valid handshake throws InvalidOperationException. This makes a connected server with no identity metadata look like a disconnected client.

This change makes McpClient.ServerInfo nullable. It returns null when a connected server omits the optional metadata, continues to return the advertised identity when present, and still throws InvalidOperationException when the client is not connected. Explicitly malformed metadata still fails during connection with JsonException.

For example, a consumer can now use client.ServerInfo?.Name after connecting to a server that omits identity metadata.

The getter keeps the same CLR return type/signature, but the nullable annotation can introduce compiler warnings for downstream C# code that dereferences ServerInfo without a null check. I chose this because it makes the existing API safe for the valid protocol response. If maintainers prefer an additive TryGetServerInfo-style API for compatibility, I'm happy to revise the PR.

Validation

• dotnet build -v quiet — passed with 0 warnings and 0 errors.
• Core client and July 2026 protocol tests (net10.0) — 43 passed.
• ASP.NET SSE and Streamable HTTP tests (net10.0) — 75 passed, 9 skipped.
• The full core test suite was started but did not complete locally; the test host stopped making progress and the run was interrupted. No full-suite pass is claimed.

Fixes #1901

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[jeffhandley]

Changing from non-nullable to nullable is technically a breaking change, so we will capture this in the release notes as such. The change looks good though and since this previously led to an exception, the breaking change is acceptable here.

Thank you both, @anushree1808 and @Coding-Professional!

[Coding-Professional]

Thanks for the reviews! Could you please rerun the failed Windows Release job? My account does not have permission to rerun it.

The build succeeded, but the .NET 9 run failed in TokenCacheTests.GetTokenAsync_ExplicitClientDoesNotRefreshTokenFromDifferentAuthorizationServer. The metadata request to https://localhost:7029/.well-known/oauth-authorization-server was canceled during the initial setup client connection (TokenCacheTests.cs:210), before the cache-validation assertions. Windows Debug and the Linux/macOS jobs passed, so this looks like a possible intermittent test-infrastructure failure unrelated to the ServerInfo changes.