Fix duplicate parameter names in introduce parameter refactoring

[Copilot]

Fix "Introduce parameter" duplicate parameter names

This PR fixes an issue where the "Introduce parameter" refactoring could generate duplicate or ambiguous parameter names when the generated name conflicts with an existing parameter.

Problem:
When using the "Introduce parameter" refactoring on an expression like v + 1 in a method that already has a parameter named v, the refactoring would generate a duplicate parameter v, leading to compiler errors:

• CS0100: The parameter name 'v' is a duplicate
• CS0229: Ambiguity between 'int v' and 'int v'

Solution:
Modified GetNewParameterNameAsync in IntroduceParameterDocumentRewriter.cs to use ISemanticFactsService.GenerateUniqueLocalName to ensure the generated parameter name doesn't conflict with existing parameters. The method now:

1. Generates a base name for the parameter (from variable declarator or expression) using a conditional expression
2. Uses GenerateUniqueLocalName which automatically determines conflicts from the expression location
3. Returns a unique name by appending numerical suffixes (v1, v2, etc.) when needed

Testing:
Added comprehensive tests covering all three code action options:

• Direct refactoring
• Trampoline method extraction
• Method overload generation

Status:

• Explore the repository and understand the issue
• Locate the relevant code in IntroduceParameterDocumentRewriter.cs
• Identify the GetNewParameterNameAsync method that generates parameter names
• Modify GetNewParameterNameAsync to use ISemanticFactsService.GenerateUniqueLocalName
• Add comprehensive tests to verify the fix works for all three code action options
• Build and verify the changes compile successfully
• Run code review - PASSED with no comments
• Run CodeQL security check - PASSED with no issues
• Add WorkItem attributes to all new tests
• Refactor to use conditional expression and simplified logic

Fixes #81400

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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[CyrusNajmabadi]

@copilot add WorkItem("https://github.com/dotnet/roslyn/issues/81400") for all tests you've added.

[Copilot]

Done in ae24b17.

[CyrusNajmabadi]

@copilot change this to a conditional expression like so:

var baseName = ShouldRemove(...)
    ? varDeclName
    : _semanticFacts.GenerateNameForExpression(...);

[Copilot]

Done in cace554.

[CyrusNajmabadi]

@copilot do you need to pass existingParameterNames along? i think GenerateUniqueName figures it out from the locations you pass in. Also, use GenerateUniqueLocalName. I don't think you need the container if you have the _expression as that's the location we can use to determine the names of things in scope.

[Copilot]

Done in cace554. Changed to use GenerateUniqueLocalName with _expression as the location and container: null, following the pattern used in IntroduceVariable. This automatically determines conflicts without needing to pass existing parameter names.