Fix form mode elicitation schema check to apply for all form requests

[Copilot]

• Changed condition from if (string.Equals(request.Mode, "form", StringComparison.Ordinal) && elicitationCapability.Form is null) to if (string.Equals(request.Mode, "form", StringComparison.Ordinal))
• Updated test FormElicitation_Requires_RequestedSchema to use Elicitation = new() instead of explicit URL-only capability
• All 33 elicitation tests pass

Original prompt

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This section details on the original issue you should resolve

<issue_title>Default RequestSchema for form mode breaking change</issue_title>
<issue_description>Prior to adding support for url elicitations, RequestedSchema defaulted to an empty object schema. Now if you don't supply a schema, elicitation requests throw. Was this an intentional breaking change?
cc: @halter73 </issue_description>

<agent_instructions>Please just add a test validating the ArgumentException discussed.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@halter73 I assume you saw [the ArgumentException](https://github.com/modelcontextprotocol/csharp-sdk/blob/774411b22c2797055fd8d2dde7c9dfaf1cc2bd52/src/ModelContextProtocol.Core/Server/McpServer.Methods.cs#L495) complaining "Form mode elicitation requests require a requested schema."? I did intentionally add that in https://github.com//pull/1021. I figured that it didn't make sense to do a form-mode elicitation without any parameters, so it's better to raise an error as quickly as possible.

It appears I didn't add a test verifying we throw this ArgumentException though. I can add that. Or do you think we should revert the behavior?</comment_new>
<comment_new>@stephentoub

I figured that it didn't make sense to do a form-mode elicitation without any parameters

I agree it's an oddity and likely indicative of a bug. As long as it was an intentional change, I think it's ok.

And we can't make the schema required because it's not necessary for url mode?</comment_new>
<comment_new>@halter73

And we can't make the schema required because it's not necessary for url mode?

Exactly.</comment_new>

• Fixes Default RequestSchema for form mode breaking change #1146

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[halter73]

Suggested change

	Elicitation = new ElicitationCapability
	{
	Url = new UrlElicitationCapability(),
	}
	Elicitation = new(),

@copilot I think it's only by chance that we check request.RequestedSchema is null before elicitationCapability.Form is null in McpServer.ThrowIfElicitationUnsupported, so we should make sure we configure the client with valid capabilities in case we ever reorder those checks which should be inconsequential.

I think this should work without explicitly declaring the Form capability, since we default to setting it if you don't set Url.

[Copilot]

The suggested change Elicitation = new() causes the test to fail because McpClientImpl defaults to setting Form = new() when both Form and Url are null (lines 108-113 in McpClientImpl.cs). With Form capability enabled, the check in ThrowIfElicitationUnsupported at line 491 (elicitationCapability.Form is null) is false, so the block containing the ArgumentException check is never entered.

The current production code only throws the "Form mode elicitation requests require a requested schema" ArgumentException when the client doesn't support form mode (Form is null). This seems to be the intended behavior based on the condition structure. The test with URL-only capability correctly tests this specific case.

Should we instead change the production code to check for missing RequestedSchema for all form mode requests, regardless of Form capability? That would require moving the schema check outside the Form is null block.

[halter73]

Should we instead change the production code to check for missing RequestedSchema for all form mode requests, regardless of Form capability? That would require moving the schema check outside the Form is null block.

Yes. But rather than moving the schema check out of the if block, I think if (string.Equals(request.Mode, "form", StringComparison.Ordinal) && elicitationCapability.Form is null) should probably just be changed to if (string.Equals(request.Mode, "form", StringComparison.Ordinal)). We check elicitationCapability.Form is null again before throwing an InvalidOperationException, so it was redundant.

[halter73]

@copilot Please make this change.

Should we instead change the production code to check for missing RequestedSchema for all form mode requests, regardless of Form capability? That would require moving the schema check outside the Form is null block.

Yes. But rather than moving the schema check out of the if block, I think if (string.Equals(request.Mode, "form", StringComparison.Ordinal) && elicitationCapability.Form is null) should probably just be changed to if (string.Equals(request.Mode, "form", StringComparison.Ordinal)). We check elicitationCapability.Form is null again before throwing an InvalidOperationException, so it was redundant.

[Copilot]

@copilot Please make this change.

Should we instead change the production code to check for missing RequestedSchema for all form mode requests, regardless of Form capability? That would require...

Done in 9a2de43. Changed the condition to if (string.Equals(request.Mode, "form", StringComparison.Ordinal)) and updated the test to use Elicitation = new(). All elicitation tests pass.