test: Add secret detection benchmark dataset and ground truth

Add comprehensive benchmark dataset with 32 documented secrets for testing
secret detection workflows (gitleaks, trufflehog, llm_secret_detection).

- Add test_projects/secret_detection_benchmark/ with 19 test files
- Add ground truth JSON with precise line-by-line secret mappings
- Update .gitignore with exceptions for benchmark files (not real secrets)

Dataset breakdown:
- 12 Easy secrets (standard patterns)
- 10 Medium secrets (obfuscated)
- 10 Hard secrets (well hidden)

test_projects/secret_detection_benchmark/config/oauth.json

@@ -0,0 +1,11 @@
+{
+  "oauth_provider": "google",
+  "client_id": "123456789012-abcdefghijklmnopqrstuvwxyz123456.apps.googleusercontent.com",
+  "client_secret": "GOCSPX-Ab12Cd34Ef56Gh78Ij90Kl12",
+  "redirect_uri": "https://example.com/oauth/callback",
+  "scopes": [
+    "openid",
+    "email",
+    "profile"
+  ]
+}