Choose unique default names for func new

[Genmin]

Fixes #3262.

Summary

• when func new offers a default function name, choose the first available name instead of defaulting to an existing artifact
• use HttpTrigger, HttpTrigger1, HttpTrigger2, ... style naming for classic folder templates and Node v4 file templates
• add unit coverage for the default-name increment helper

Validation

• git diff --check
• Not run locally: this machine does not have the .NET SDK (dotnet, csc, and msbuild are unavailable).

[Copilot]

Pull request overview

Updates func new to avoid suggesting a default function name that would collide with an existing function artifact, aligning behavior with VS Code’s “auto-increment” naming.

Changes:

• Compute a unique default function name before prompting (e.g., HttpTrigger, HttpTrigger1, HttpTrigger2, …).
• Add GetUniqueDefaultFunctionName helper and artifact-existence check for classic (directory) vs Node v4 (file) templates.
• Add unit tests covering the incrementing behavior of the helper.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Cli/func/Actions/LocalActions/CreateFunctionAction.cs	Introduces unique default-name selection and artifact-existence checks for traditional vs Node v4 templates.
test/Cli/Func.UnitTests/ActionsTests/CreateFunctionActionTests.cs	Adds unit coverage for the default-name increment helper.

[liliankasem]

When FileName is set, FunctionArtifactExists ignores the candidate name and only checks the fixed file path. If that file already exists, every candidate returns true and GetUniqueDefaultFunctionName loops forever, hanging func new before it ever prompts. Easiest fix is to skip the uniqueness loop when FileName != null (renaming the function can't avoid a fixed-file collision anyway).

Cleanest fix:

var defaultFunctionName = FileName != null
    ? template.Metadata.DefaultFunctionName
    : GetUniqueDefaultFunctionName(
        template.Metadata.DefaultFunctionName,
        functionName => FunctionArtifactExists(functionName, FileName, template));

[liliankasem]

This exact code/check already exists inlined in TemplatesManager.cs:145 - you should extract this a shared helper instead of duplicating

[liliankasem]

Right now this PR only covers the traditional/Node v4 path, .NET (in-proc and isolated) and the Python v2 programming model take their own branches earlier in RunAsync and still silently prompt with a colliding default. Please extend the uniqueness behavior to those two paths as well.

• .NET (in-proc & isolated) branch: lines 144–162 (if (WorkerRuntimeLanguageHelper.IsDotnet(_workerRuntime) && !Csx)), default name is set at line 158 (FunctionName = string.IsNullOrEmpty(FunctionName) ? TemplateName : FunctionName;).

• Python v2 (new programming model) branch: lines 165–234 (else if (IsNewPythonProgrammingModel())), default name is resolved at lines 227–230 (if (string.IsNullOrEmpty(FunctionName)) FunctionName = providedInputs[GetFunctionNameParamId];).

Both bypass the new GetUniqueDefaultFunctionName call

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