Co-authored-by: sbomer <[email protected]>

#121861)

Backport of #121603 to
release/10.0

## Customer Impact

- [x] Customer reported
- [ ] Found internally

Reported by customer in #121546

## Regression

- [x] Yes
- [] No

Regressed in .NET 10 by analyzer changes that touched related code.

## Testing

Verified on the provided repro.

## Risk

Low. Adds an early return to the analysis in a place that doesn't have
downstream effects.


<hr />


Backports #121603 to release/10.0.
Fixes #121546.

The RoslynAnalyzer crashes when analyzing indexer property parameter
initializers because it assumes all parameters belong to methods, but
Roslyn provides operation blocks for property indexer parameters that
don't have a direct IL equivalent.

## Changes

- **TrimAnalysisVisitor.cs**: Guard against null `parameterMethod` in
`GetParameterTargetValue`. Returns `TopValue` for parameters not owned
by methods (e.g., indexer parameter initializers), preventing crashes
while preserving correct analysis for method parameters.

- **PropertyDataFlow.cs**: Add test coverage for annotated indexer
parameters and indexers with default arguments to verify the analyzer
handles these cases without crashing.

```csharp
// Previously crashed on this pattern:
public Type this[[DynamicallyAccessedMembers(...)] Type index]
{
    get { ... }
    set { ... }
}
```

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: sbomer <[email protected]>