Add type annotations to SIP2 client and provider

[jonathangreen]

Description

Add comprehensive type annotations to the SIP2 authentication client and provider modules, and refactor the SIP2 field descriptor system to use idiomatic Python patterns.

Motivation and Context

The SIP2 modules were excluded from mypy type checking. This change adds full type coverage, enabling static analysis to catch bugs in this critical authentication path. The field descriptor refactoring replaces a non-standard _add() pattern with standard Python descriptors, improving readability and IDE support.

How Has This Been Tested?

• Existing SIP2 client and provider tests updated to reflect the new username: str contract.
• Removed tests for None username cases that are no longer reachable.

Checklist

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

[codecov]

Codecov Report

❌ Patch coverage is 94.19355% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.18%. Comparing base (cb46a97) to head (cd4f7af).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
...ace/manager/integration/patron_auth/sip2/client.py	96.24%	4 Missing and 1 partial ⚠️
...e/manager/integration/patron_auth/sip2/provider.py	80.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3091      +/-   ##
==========================================
- Coverage   93.20%   93.18%   -0.02%     
==========================================
  Files         491      491              
  Lines       45248    45256       +8     
  Branches     6222     6224       +2     
==========================================
  Hits        42172    42172              
- Misses       1992     1997       +5     
- Partials     1084     1087       +3     

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[dbernstein]

Looking good.