Fix duplicate parameter names in introduce parameter refactoring

[Copilot]

The "Introduce parameter" refactoring generated duplicate parameter names when the inferred name conflicted with existing parameters, causing CS0100 and CS0229 compiler errors.

Changes

• Modified GetNewParameterNameAsync in IntroduceParameterDocumentRewriter.cs to use ISemanticFactsService.GenerateUniqueName, ensuring generated parameter names don't conflict with existing parameters by appending numerical suffixes when needed

• Added test coverage for all three code action variants (direct refactoring, trampoline, overload) and edge cases including multiple conflicts

Example

Before:

int M(int v) => v + 1;  // Select "v + 1" → Introduce parameter
// Generated: int M(int v, int v) { }  // CS0100: duplicate parameter

After:

// Generated: int M(int v, int v1) { }

Original prompt

---

This section details on the original issue you should resolve

<issue_title>"Introduce parameter" may generate duplicate/ambiguous parameters</issue_title>
<issue_description>Version Used: VS 26 Insiders [11218.70]

Steps to Reproduce:
Select the marked range, open the lightbulb menu and "Introduce parameter for v+1" -> "into new overload"

class C
{
    int M(int v)
    {
        return $v + 1$;
    }
}

Expected Behavior:
The new parameter name is unambiguous and not a duplicate, e.g. by adding a numerical suffix, like v1.

Actual Behavior:
The new parameter has the same name as the existing parameter, leading to conflicts:

int M(int v, int v) // CS0100: The parameter name 'v' is a duplicate
{
    return v; // CS0229: Ambiguity between 'int v' and 'int v'
}

All three options for "Introduce parameter" have this issue in some way.</issue_description>

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

• Fixes "Introduce parameter" may generate duplicate/ambiguous parameters #81400

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