Fix error return values for no-op UI_xxx stub functions

[justsmth]

Description of changes:

AWS-LC provides no-op stub implementations of several OpenSSL UI_xxx functions to allow compilation of projects that reference them for non-essential operations. These stubs always fail at runtime since the UI API is unsupported.

Previously, UI_add_input_string, UI_add_verify_string, UI_add_info_string, and UI_process all returned 0. In OpenSSL's UI API, -1 indicates failure:

• UI_add_input_string, UI_add_verify_string, and UI_add_info_string all delegate to general_allocate_string(), which returns a positive index on success and -1 on error.
• UI_process returns 0 on success and -1 on error.

In practice, callers should already be checking the NULL return from UI_new and never reaching these functions. This change simply corrects the return values to be consistent with the OpenSSL API contract, and updates the corresponding header documentation to match.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.34%. Comparing base (167015c) to head (a2d714a).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crypto/ui/ui.c	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3025      +/-   ##
==========================================
+ Coverage   78.32%   78.34%   +0.01%     
==========================================
  Files         689      689              
  Lines      121007   121010       +3     
  Branches    16994    16992       -2     
==========================================
+ Hits        94783    94807      +24     
+ Misses      25330    25306      -24     
- Partials      894      897       +3     

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