feat: 13 e2e integration tests against real Mistral API

Orchestrator tests:
- Simple chat roundtrip with token usage verification
- Event ordering (Started → Thinking → Done)
- Metadata pass-through (opaque bag appears in Started event)
- Token usage accuracy (longer prompts → more tokens)
- Conversation continuity (multi-turn recall)
- Client-side tool dispatch + mock result submission
- Failed tool result handling (is_error: true)
- Server-side tool execution (search_web via conversation)

gRPC tests:
- Full roundtrip (StartSession → UserInput → Status → TextDone)
- Client tool relay (ToolCall → ToolResult through gRPC stream)
- Token counts in TextDone (non-zero verification)
- Session resume (same room_id, resumed flag)
- Clean disconnect (EndSession → SessionEnd)

Infrastructure:
- ToolRegistry::new_minimal() — no OpenSearch/Matrix needed
- ToolRegistry fields now Option for testability
- GrpcState.matrix now Option
- grpc_bridge moved to src/grpc/bridge.rs
- TestHarness loads API key from .env

Cargo.toml

@@ -45,6 +45,9 @@ tokio-stream = "0.1"
 jsonwebtoken = "9"
 tokenizers = { version = "0.22", default-features = false, features = ["onig", "http"] }
 
+[dev-dependencies]
+dotenv = "0.15"
+
 [build-dependencies]
 tonic-build = "0.14"
 tonic-prost-build = "0.14"