Refactored Instrument Retrieval and Contract Master Logic for Improved Performance and Maintainability

[parva2612]

This PR introduces a complete refactor of the contract master download and instrument lookup logic to enhance efficiency, maintainability, and performance.

Key Improvements:

Centralized Master Contract Download:

• Unified function download_master_contract() downloads and consolidates data across all exchanges.
• Optional CSV export for persistence or debugging.

Optimized In-Memory Filtering:

• Contract data is stored in-memory using pandas.DataFrame, eliminating repeated file I/O operations.
• Fast and efficient lookup using pre-indexed dataframes (master_contract_df_symbol and master_contract_df_token).

Cleaned and Standardized Data:

• Normalized column names and structure across all exchanges.
• Expiry dates parsed and numeric fields (instrument_token, strike, lot_size) cast to correct types.
• Dropped duplicates and null rows to ensure data consistency.

Improved Instrument Retrieval:

• Simplified and optimized:
  • get_instrument_by_symbol()
  • get_instrument_by_token()
  • get_instrument_for_fno()
• Enhanced validation (e.g., expiry presence, correct strike type).
• Handles multiple instruments gracefully when applicable.

Backward Compatibility:

• Returns Instrument objects consistent with the previous implementation.
• Ensures compatibility with existing systems while improving overall structure.

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Let me know if you'd like to include benchmarks, unit tests, or additional documentation updates before merging.