fix: improve json handling and response parsing in textToJSON class

[KRISHNA-12082006]

Summary

This PR improves backend stability and correctness by:

1. Preventing shared mutable default state in textToJSON.__init__.
2. Making the answer‑to‑field mapping deterministic by using definitions.
3. Adding safe handling for Ollama response status and missing data.
4. Cleaning up plural value parsing logic.

Motivation

• Prevents hidden bugs where repeated calls leak data.
• Ensures each field gets the right answer.
• Avoids crashes when the Ollama server errors.
• Simplifies plural value parsing for consistent output.

Manual Validation

• Ran python src/main.py with sample templates.
• Verified consistent answers in filled PDFs.
• Ensured error is raised when Ollama is unreachable.