`GET /api/settings/{key}` was calling `validate_setting_key()` which raises
HTTP 404 for keys not in SETTING_METADATA. The test expects 200 with value=None
for unknown keys.
Added `validate_setting_key_format()` to `input_validation.py` that validates
only the key format without the SETTING_METADATA existence check. Updated
`get_setting` to use the format-only validator; POST/DELETE endpoints continue
using the full `validate_setting_key()` for write-side security.
Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
copilot-swe-agent[bot]
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