Fix AST processor tests: handle files.LanguageContent type assertions

[bernardoforcillo]

This PR fixes the skipped tests TestASTProcessorErrorHandling and TestASTProcessorWithNestedObjects in internal/processor/tests/processor_deep_test.go.

The tests were failing because they asserted that nested maps in the result were of type map[string]interface{}, but ASTProcessor.Execute returns files.LanguageContent (which is a defined type based on map[string]interface{}).

Changes:

• Unskipped TestASTProcessorErrorHandling and TestASTProcessorWithNestedObjects.
• Added a helper function castToLanguageContent to robustly cast values to files.LanguageContent, handling both files.LanguageContent and map[string]interface{} types.
• Refactored the tests to use this helper function.

This ensures that the tests correctly verify the error handling and nested object processing behavior of the AST processor.

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PR created automatically by Jules for task 18084114665363459455 started by @bernardoforcillo

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