Fix multiple code quality and validation issues

[xenacode-art]

Hi @zache-fi @ad-an-26 ,
This commit addresses several bugs and code quality issues found during code review:

1. Add missing validator to superseded_similarity_threshold field

   • Field documentation stated 0.0-1.0 range but had no validator
   • Added MinValueValidator and MaxValueValidator for consistency
   • All other similar ORES threshold fields already had validators

2. Fix inconsistent exception handling in is_living_person.py

   • Changed return value from True to False when birth date check fails
   • Added proper logging for debugging
   • Ensures consistent behavior with other exception handlers

3. Add input validation to ISBN checker functions

   • Added None/empty string guards to prevent index out of bounds
   • Prevents potential crashes when invalid input is provided
   • Improves defensive programming practices

4. Improve exception specificity in pending_revision.py

   • Split broad Exception catches into specific pywikibot exceptions
   • Now handles APIError, ServerError, and NoPageError separately
   • Better error logging for API vs unexpected errors
   • Enables more targeted error handling and monitoring

5. Document and resolve wikitext TODO comment

   • Replaced TODO with clear explanation of str() conversion
   • Documents that MediaWiki API can return bytes or other types
   • Clarifies why explicit string conversion is necessary

All changes have been tested and pass existing test suite (70 tests).