[proxy] LLM parsers, pricing, and builtin middlewares (OpenAI, Anthropic, Vertex AI, AWS Bedrock)

Request/response parsers and SSE/event-stream metering, the embedded pricing
table, and the builtin middleware set: request parser, router, policy
limit-check/record, cost meter, guardrail, identity inject, response parser.
Includes the path-routed providers — Google Vertex AI (keyfile:: service-account
OAuth minting) and AWS Bedrock (bearer auth, invoke/converse/streaming, optional
/bedrock prefix) — plus the Models allowlist and unmeterable-publisher deny.

proxy/internal/llm/fixtures/openai_chat_completion.json

@@ -0,0 +1,21 @@
+{
+  "id": "chatcmpl-abc",
+  "object": "chat.completion",
+  "created": 1700000000,
+  "model": "gpt-4o-mini",
+  "choices": [
+    {
+      "index": 0,
+      "message": {
+        "role": "assistant",
+        "content": "Hello, world!"
+      },
+      "finish_reason": "stop"
+    }
+  ],
+  "usage": {
+    "prompt_tokens": 123,
+    "completion_tokens": 45,
+    "total_tokens": 168
+  }
+}