ash config validate accepts multi-line `reason` in suppressions, breaking markdown report rendering

[rafaelpereyra]

Summary

A suppression entry with a newline character in its reason field passes ash config validate with no warning, but breaks rendering in the generated ash.summary.md and ash.unused-suppressions.md reports (the reason is placed into a markdown table cell, which doesn't allow embedded newlines).

Reproduction

1. Add a suppression with a multi-line reason to .ash/.ash.yaml:

   global_settings:
     suppressions:
       - path: "src/foo.py"
         rule_id: "B201"
         reason: |
           Multi-line reason
           with a newline.

2. Run ash config validate → reports ✅ Configuration is valid!
3. Run a scan and inspect .ash/ash_output/reports/ash.summary.md — the suppression row in the table is broken / split across lines.

Expected

ash config validate (and/or ash config lint) should emit a warning when a suppression reason contains a newline, since this is reliably a downstream rendering hazard. Auto-fix could collapse newlines to spaces.

Notes

• The AshSuppression model in automated_security_helper/models/core.py declares reason: str (inherited from IgnorePathWithReason) with no content validation.
• ConfigValidator.validate_config_file in automated_security_helper/config/config_validator.py only validates structural concerns (top-level fields, internal-only fields, path patterns) and does not inspect suppression reason content.
• Suggested fix location: add a check in ConfigLinter._check_suppression_issues (automated_security_helper/config/config_linter.py) emitting a WARNING (e.g. a new SUPPRESSION_REASON category), and consider an auto-fix that replaces \n with a single space.