refactor: standardize prompt parameter naming and enhance benchmark base class

- Changed all prompt parameters from '{text}' to '{input}' for consistency
- Enhanced Benchmark base class with prompt loading and validation
- Added test data loading functionality with proper error handling
- Improved initialization to accept prompt path and test data directory
- Added validation for prompt format and file existence
- Implemented structured test data loading from directory
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The commit message follows conventional commit format with a clear title and descriptive body explaining the changes and their purpose.

prompts/custom.txt

@@ -0,0 +1 @@
+{input}