Add type annotations to Credential model methods

[jonathangreen]

Description

Add comprehensive type annotations to the Credential model methods and enable strict mypy checking for the module.

Key changes:

• Add type hints to lookup, lookup_by_token, lookup_and_expire_temporary_token, temporary_token_create, persistent_token_create, and __repr__ methods
• Make token and token_type parameters non-nullable where semantically appropriate (looking up by None values is logically a bug)
• Fix classmethod using self instead of cls in persistent_token_create
• Fix incorrect indentation in lookup_by_patron return statement

Motivation and Context

This change improves type safety in the Credential model by:

1. Enabling strict mypy checking (removing the module from the exclusion list)
2. Making the API more explicit about what values are expected
3. Catching potential bugs at type-check time rather than runtime

How Has This Been Tested?

• All existing tests pass
• mypy passes with no errors across the entire codebase (1017 source files)
• Added test assertions for type narrowing on nullable fields

Checklist

• I have updated the documentation accordingly.
• All new and existing tests passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.81%. Comparing base (d8b8dc1) to head (c9871b8).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2977   +/-   ##
=======================================
  Coverage   92.81%   92.81%           
=======================================
  Files         454      454           
  Lines       42942    42942           
  Branches     5999     5999           
=======================================
  Hits        39857    39857           
  Misses       2016     2016           
  Partials     1069     1069           

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[tdilauro]

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