OpenAI Agents SDK Guardrail Span Classification Issues

# OpenAI Agents SDK Guardrail Span Classification Issues

## Problem Description

The OpenAI Agents SDK integration has issues with guardrail span classification and duplicate span creation:

1. **Incorrect Span Classification**: Guardrail events were being incorrectly classified as generic events or agent spans instead of proper `GUARDRAIL` span types
2. **Duplicate Span Creation**: Each guardrail creates 2 spans: `{guardrail_name}` and `{guardrail_name}.guardrail`, making it unclear which span represents the actual guardrail execution
3. **Lost Usage Metrics**: Recent fixes removed the wrong span, causing usage metrics to be lost from guardrail spans

## Root Cause

Two functions in the OpenAI Agents SDK instrumentation were missing handlers for `GuardrailSpanData`:

1. `get_span_kind()` in `exporter.py` - didn't handle `GuardrailSpanData`
2. `get_span_attributes()` in `attributes/common.py` - didn't have a case for `GuardrailSpanData`

## Expected Behavior

- Guardrail events should be properly classified as `GUARDRAIL` spans
- Each guardrail should create only one span with proper usage metrics
- Guardrail spans should have appropriate attributes and observability data

## Current Status

PR #1096 attempted to fix this but introduced new issues with duplicate spans and lost usage metrics.

## Acceptance Criteria

- [ ] Guardrail events are properly classified as `GUARDRAIL` span type
- [ ] No duplicate spans are created for guardrails
- [ ] Usage metrics are preserved in guardrail spans
- [ ] Proper attributes are set for guardrail spans
- [ ] Integration tests verify correct behavior

## Related

- PR #1096: Fix OpenAI Agents SDK guardrail span classification