fix: register exception handlers and fix handle_plural_values() white spaces

[vidhip222]

Two bug fixes to the LLM data extraction pipeline and API error handling.

1. src/llm.py: Fix whitespace stripping in handle_plural_values()
   The previous loop started at index i+1, leaving values[0] (the first LLM-extracted value) never stripped. It also only applied lstrip(), missing trailing whitespace. Replaced with a list comprehension using str.strip() that correctly cleans all values and filters out empty entries from trailing semicolons. Also adds type hints (str -> list[str]) and fixes a typo in the log message.
2. api/main.py: Register custom exception handlers
   Register_exception_handlers() was defined in api/errors/handlers.py but never called on app startup, so AppError exceptions (e.g. 404 for missing templates) were unhandled by FastAPI and returned as 500 errors instead of the intended status codes.

Issues:
Fixes #283
Fixes #295

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Manually verified handle_plural_values() edge cases: leading whitespace, trailing whitespace, trailing semicolons, normal input
• Verified AppError with 404 status now returns correct response instead of 500

Test Configuration:

• Python: 3.x
• OS: Windows 11

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes