APICompat misses when you remove all defaults from param list

### 🔧 Tool Name

ApiCompat

### 📄 What Should This Tool Do?

If you have a method like this

```c#
public void Foo(string a = default, string b = default)
```

And you change it to this

```c#
public void Foo(string a, string b)
```

ApiCompat or some other tooling during the PR should flag this as a breaking change.

### 📥 Inputs and 📤 Outputs

dotnet build

### 🏗️ Execution Context

MCP only

### 💻 CLI Command and Flags (if applicable)

_No response_

Start by locating ApiCompat's compatibility checks for method parameter defaults and run the provided dotnet build input. Reproduce the two Foo signatures, then verify that removing all defaults is reported as a breaking change during compatibility analysis.

Written by the indexing model from the issue text.