fix: avoid double-encoding alert URLs

[aditya-786]

Closes #4737

📑 Description

Alert URL validation encoded the % in existing %HH sequences each time an alert DTO was reconstructed. It also encoded #, which broke hash-based URLs such as Wazuh agent links.

This change preserves valid percent escapes and fragments while encoding malformed percent signs exactly once. It adds regression coverage for encoded Unicode, hash fragments, raw unsafe characters, scheme-less URLs, raw Unicode paths, and malformed percent escapes, including reconstruction idempotence.

✅ Checks

• My pull request adheres to the code style of this project
• My code requires changes to the documentation
• I have updated the documentation as required
• All the tests have passed

ℹ Additional Information

Validation completed:

• pytest tests/test_alert_dto.py -k url_encoding_is_idempotent --noconftest -o addopts='' -q
• pytest tests/test_alert_dto.py -k 'not test_alert_started_at' -o addopts='' -q
• ruff check keep/api/models/alert.py tests/test_alert_dto.py
• isort --check-only keep/api/models/alert.py tests/test_alert_dto.py
• black --check tests/test_alert_dto.py

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.43%. Comparing base (98dbd7e) to head (9ef07de).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
keep/api/models/alert.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6653       +/-   ##
===========================================
+ Coverage   30.49%   46.43%   +15.93%     
===========================================
  Files         101      178       +77     
  Lines       11741    18694     +6953     
===========================================
+ Hits         3581     8681     +5100     
- Misses       8160    10013     +1853     

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[shahargl]

lgtm