feat: add OpenTelemetry tracing support

[wingding12]

Summary

Add opt-in OpenTelemetry tracing support via the tracer parameter in ClaudeAgentOptions. This enables observability for applications using the Claude SDK.

Changes

• types.py: Add tracer parameter to ClaudeAgentOptions
• _internal/tracing.py (new): Create tracing helper module with TracingContext class and span utilities
• _internal/query.py: Add tracing for control protocol requests, permission checks, hooks, and MCP calls
• _internal/client.py: Add tracing for query processing with span attributes for results
• client.py: Add session-level tracing for ClaudeSDKClient
• pyproject.toml: Add telemetry optional dependency (opentelemetry-api>=1.20.0)
• tests/test_tracing.py (new): Add comprehensive unit tests

Span Names

When tracing is enabled, the following spans are created:

Span Name	Description
claude_agent_sdk.query	Query function calls
claude_agent_sdk.session	ClaudeSDKClient sessions
claude_agent_sdk.control	Control protocol requests
claude_agent_sdk.permission	Tool permission checks
claude_agent_sdk.hook	Hook callbacks
claude_agent_sdk.mcp	MCP server requests

Usage

from opentelemetry import trace
from claude_agent_sdk import query, ClaudeAgentOptions

tracer = trace.get_tracer("my-app")
options = ClaudeAgentOptions(tracer=tracer)

async for message in query(prompt="Hello", options=options):
    print(message)

Install with: pip install claude-agent-sdk[telemetry]

Test Plan

• All existing tests pass
• New tracing tests pass (18 tests)
• Ruff linting passes
• Works correctly when opentelemetry is not installed (graceful degradation)

Closes #452