Add example: discover available MCP tools via SystemMessage init

[ZainAli24]

Problem

The MCP documentation page at
https://code.claude.com/docs/en/agent-sdk/mcp#discover-available-tools
shows only a TypeScript code snippet for discovering available MCP tools.
There is no Python equivalent — all other sections on that page offer a
Python / TypeScript toggle, but this section does not.

Following the TypeScript pattern directly in Python leads to a runtime
error because SystemMessage does not expose a .type attribute:

# This raises AttributeError: 'SystemMessage' object has no attribute 'type'
if message.type == "system" and message.subtype == "init":
    ...

Solution

This PR adds examples/mcp_discover_available_tools.py, a self-contained
example that shows the correct Python idiom for inspecting MCP server
status and available tools at session start.

What the example covers

• Connecting to an HTTP MCP server (ClaudeAgentOptions.mcp_servers)
• Detecting the SystemMessage(subtype="init") event using isinstance()
• Reading message.data["mcp_servers"] to check per-server connection
  status ("connected", "pending", "failed", "needs-auth")
• Filtering message.data["tools"] to list only MCP tools
  (mcp__<server>__<tool> naming convention)

Key correctness note documented in the example

# WRONG — raises AttributeError
if message.type == "system" and message.subtype == "init":

# CORRECT — use isinstance() to check message type
if isinstance(message, SystemMessage) and message.subtype == "init":

Testing

Run the example directly (requires a valid Claude Code session):

python examples/mcp_discover_available_tools.py

Expected output:

MCP server connection status:
  ✓ claude-code-docs: connected

Available MCP tools (N):
  - mcp__claude-code-docs__<tool_name>
  ...

Result:
<Claude's response listing available tools>