Clean up InvokeMethodExpression.Accept() and confirm ExpressionStatement/InvokeMethodExpression visitor support

[Copilot]

Addresses the remaining visitor implementation work for ExpressionStatement and InvokeMethodExpression — both were added in PR #7313 and are confirmed complete. Scoped per issue guidance to only these two types.

Changes

• InvokeMethodExpression.Accept(): Removed 6 redundant no-op self-assignments (invokeMethod.MethodName = invokeMethod.MethodName, etc.) that were left over from the initial implementation. Only InstanceReference and Arguments need updating since they are the visitable child nodes.
• Changeset: Added entry for @typespec/http-client-csharp.

The visitor hooks (VisitExpressionStatement, VisitInvokeMethodExpression) and their corresponding Accept implementations are fully functional with existing test coverage in ExpressionVisitorTests.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add remaining visit implementations for expressions/statements</issue_title>
<issue_description>#7313 did an initial pass at the statements/expressions needed to unblock the LRO work, but we should fill out the Accept/Visit methods for the remaining statements/expressions.</issue_description>

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

@JoshLove-msft Trim down the list of Visit methods to have only the ExpressionStatement and InvokeMethodExpression methods. We can add more as they are needed.

• Fixes Add remaining visit implementations for expressions/statements #7366

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