Add advanced descriptors and molecule design capabilities

[google-labs-jules]

• Enhanced PreprocessingAgent in src/agents/preprocessing.py to support extended descriptors and fingerprints (Morgan ECFP4).
• Created GenerationAgent in src/agents/generation.py to design new molecular entities using SMARTS-based reaction rules.
• Updated ModelBuilderAgent in src/agents/modeling.py with a predict method that handles feature alignment for new data.
• Updated src/main.py to integrate the new capabilities via --descriptors and --design flags.

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PR created automatically by Jules for task 5960999302576423193 started by @Abmanna

[google-labs-jules]

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