Clarify null semantics in ToolAnnotations hint property docs

[Copilot]

ToolAnnotations hint properties (DestructiveHint, IdempotentHint, OpenWorldHint, ReadOnlyHint) are typed bool? with a .NET default of null, but their XML docs said "The default is true/false" — implying a .NET-level default that doesn't exist. The documented value is the MCP spec's prescribed interpretation when the property is absent.

Changes

• ToolAnnotations: Updated <value> docs on all four hint properties to enumerate all three states (true, false, null) and explicitly state the spec-prescribed assumption clients should make when the value is null.

Example — DestructiveHint before:

The default is true.

After:

true if the tool can perform destructive updates to its environment;
false if the tool performs only additive updates;
null if unspecified, in which case clients should assume true.

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