🧪 [testing] add unit tests for normalizeInstr

[julesklord]

I have added unit tests for the normalizeInstr function to improve the reliability and coverage of the codebase.

🎯 What

Addressed the testing gap for the normalizeInstr utility function which is responsible for mapping various instrument names to internal codes.

📊 Coverage

The new tests cover the following scenarios:

• Edge cases: null, undefined, strings, objects, and empty arrays (all default to ['g']).
• Mapping: Correctly maps synonyms like 'guitarra' to 'g', 'keyboards' to 'p', 'horns' to 'v', etc.
• Codes: Accepts existing internal codes ('g', 'p', 'v', 'o').
• Normalization: Handles case-insensitivity ('GUITAR') and leading/trailing spaces (' Vocals ').
• Deduplication: Removes duplicate codes if multiple input names map to the same code.
• Validation: Filters out unrecognized instrument names.
• Defaulting: Returns ['g'] if no valid instruments remain after filtering.

✨ Result

• Improved test coverage for core utility functions.
• Increased confidence in instrument normalization logic during CSV imports and song creation.
• All 33 automated tests are passing.

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PR created automatically by Jules for task 13315948952698931644 started by @julesklord

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