feat(02-01): add Google AI, Vertex AI, AWS Bedrock, xAI providers

- google-ai: AIzaSy pattern for Gemini
- vertex-ai: AIzaSy + Bearer verify on aiplatform endpoint
- aws-bedrock: ABSK long-token and AKIA medium patterns
- xai: xai- 80-char token pattern
- All dual-located in providers/ and pkg/providers/definitions/

providers/xai.yaml

@@ -0,0 +1,20 @@
+format_version: 1
+name: xai
+display_name: xAI (Grok)
+tier: 1
+last_verified: "2026-04-05"
+keywords:
+  - "xai-"
+  - "xai"
+  - "grok"
+patterns:
+  - regex: 'xai-[0-9a-zA-Z_]{80}'
+    entropy_min: 3.5
+    confidence: high
+verify:
+  method: GET
+  url: https://api.x.ai/v1/api-key
+  headers:
+    Authorization: "Bearer {KEY}"
+  valid_status: [200]
+  invalid_status: [401, 403]