[Tests] AISettingsProvider — test DB→config→env→throw fallback chain

[fboucheros]

What to build

AISettingsProvider.GetAISettingsAsync() contains non-trivial fallback logic (DB Settings → IConfiguration → environment variable → throw) with zero tests. Add unit tests for each branch.

Acceptance criteria

• Test: API key found in DB Settings.AiApiKey → returned successfully
• Test: DB key absent → falls back to IConfiguration value → returned successfully
• Test: DB + config absent → falls back to environment variable → returned successfully
• Test: all three sources absent → throws expected exception
• Mocks use IDataStorageService and IConfiguration (no real infrastructure needed)
• All new tests pass in CI

Blocked by

None — can start immediately.

Priority

🔴 Critical — fallback logic is critical for configuration correctness

[fboucher]

Triage: Assigned to Linus (Tester).
Branch: squad/103-ai-settings-provider-fallback
Scope: Test AISettingsProvider.GetAISettingsAsync() fallback chain (DB → config → environment → throw) with zero coverage.