Scattered test credential strings (API keys, OAuth tokens, crypto keys,
Telegram tokens, session tokens) across ~25 files made security auditing
harder and created unnecessary duplication. Centralize all test-only fake
credentials into a new `src/testing/credentials.rs` module with named
constants and a shared `test_secrets_store()` helper.
- Convert `src/testing.rs` to directory module (`src/testing/mod.rs`)
- Add `src/testing/credentials.rs` with ~30 named constants
- Replace hardcoded literals in 24 source files
- Deduplicate `test_store()` helper (was copy-pasted in 3 files)
- Leave leak_detector/shell/signature tests as-is (inline values
aid readability for pattern detection tests)
[skip-regression-check]
Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>