Backport PR #121603: Fix RoslynAnalyzer crash on indexer property parameters

[Copilot]

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.

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

Original prompt

Backport #121603 to the release/10.0 branch.

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