PROMPT 10 Error And Notification Infrastructure

# PROMPT_10: Error and Notification Infrastructure

**Goal:** Implement `McpClient.Error` and `McpClient.NotificationRouter` modules to support client features.

**Duration:** ~30 minutes

**Dependencies:** PROMPT_01-09 (Core complete)

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## Context

Before implementing high-level MCP feature APIs (Tools, Resources, etc.), we need shared infrastructure for:

1. **Error normalization** - Convert various error types into structured `McpClient.Error` structs
2. **Notification routing** - Route server-initiated notifications to typed callbacks

These modules provide the foundation for consistent error handling and notification dispatch across all feature modules.

> **Reminder (ADR-0015):** Notification routing remains synchronous in the MVP. Build the router so future work can optionally fan out via a `Task.Supervisor`, but do not introduce async processing yet.

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## Required Reading

**ADR-0011:** Client Features Architecture
- Section: "Shared Infrastructure"
- Error normalization strategy
- Notification routing pattern

**PROTOCOL_DETAILS.md:**
- Section: "Error Codes" - JSON-RPC and MCP error codes
- Section: "Message Methods" - Notification methods

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## Implementation Requirements

### 1. McpClient.Error Module

**File:** `lib/mcp_client/error.ex`

**Purpose:** Normalized error type for all MCP operations.

**Implementation:**