feat(sep-2200): align tool result content visibility with SEP-2200

Closes #1552

Summary

Brings the C# SDK into compliance with SEP-2200 — "Clarify tool result content visibility"

SEP-2200 clarifies that CallToolResult.content is model-oriented (may be a prose summary) and CallToolResult.structuredContent is machine-oriented (strict JSON for programmatic consumers). Clients SHOULD prefer content for model context and SHOULD NOT forward both fields verbatim to the LLM. The wire format is unchanged.

What

Behavioural fix McpClientTool.InvokeCoreAsync

Removed result.StructuredContent is null from the gate. The Content-block conversion path now triggers whenever there's no protocol-level info to preserve (IsError=true or non-empty Meta), regardless of whether StructuredContent is also set.

See THIS comment for more info.

- if (result.IsError is not true &&
-     result.StructuredContent is null &&
-     result.Meta is not { Count: > 0 })                                                                                                                                                                        
+ if (result.IsError is not true &&
+     result.Meta is not { Count: > 0 })                                                                                                                                                                        

XML doc updates (no behaviour change)

• CallToolResult.Content / StructuredContent — model-vs-machine semantics + the SEP-2200 selection rule.
• [McpServerTool].UseStructuredContent / OutputSchemaType — call out the default JSON-dump behaviour and point to the recommended decoupled-fields pattern.
• McpServerToolCreateOptions.UseStructuredContent / OutputSchema — same wording mirrored on the options surface.
• McpClient.CallToolAsync — <remarks> explaining which field to forward to the model and which to consume programmatically.
  ### Sample
  New samples/EverythingServer/Tools/WeatherStructuredTool.cs demonstrating the SEP-2200 recommended pattern: returns a CallToolResult with model-friendly prose in Content ("It's 72°F and sunny in Paris …") alongside machine-friendly JSON in StructuredContent ({"city":"Paris","tempF":72,…}), with the schema advertised via OutputSchemaType = typeof(WeatherReading).

Behavioural change call-out

Clients embedding this SDK in IChatClient flows will stop double-feeding content + structuredContent to the LLM.
The default [McpServerTool(UseStructuredContent=true)] JSON-dump-into-both shape is preserved (SEP-2200 explicitly permits it, though considers it suboptimal).

No wire change

SEP-2200 itself is non-wire-breaking: same content, same structuredContent, same types. Only the SDK's consumption of the wire result changed.

Test plan

• dotnet test tests/ModelContextProtocol.Tests (net10.0): 1887 passed, 0 failed, 5 skipped.
• Targeted: red-then-green confirmed for StructuredContentTool_PrefersContentBlocksForModel
• dotnet build src/ModelContextProtocol.Core and dotnet build samples/EverythingServer: 0 warnings, 0 errors across all target frameworks.