Add configurable default for sampling MaxTokens

[Copilot]

• Add DefaultSamplingMaxTokens property to McpServerOptions with a default value of 1000 tokens
• Update sampling code in McpServer.Methods.cs to use the default instead of int.MaxValue
• Fix test to mock ServerOptions property
• Augment existing test to verify DefaultSamplingMaxTokens is respected by SampleAsync method
• Build and test the changes - all 719 tests passing

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[stephentoub]

What is this addressing / what is the impetus for this?

[MackinnonBuck]

@stephentoub The CreateMessageRequestParams protocol type had an incorrectly optional MaxTokens property (the schema shows it as being required). #892 changes MaxTokens from an int? to a required int, but this means a value needs to be specified where it didn't previously. To avoid restructuring APIs like McpServer.SampleAsync(...) to always require a MaxTokens value, this new DefaultSamplingMaxTokens option controls the default.

See also #892 (comment) and #892 (comment)