Document ASGI-only requirement for BedrockAgentCoreContext

## Summary

BedrockAgentCoreContext uses ContextVar for storing request-scoped sensitive data (workload access tokens, OAuth2 callback URLs, authorization headers). While this is safe for ASGI web frameworks, it creates a **critical security vulnerability** if used with WSGI frameworks.

## Problem

**ContextVar isolation behavior:**
- ✅ **ASGI frameworks** (Starlette, FastAPI, Quart): Automatically isolated per async task (each HTTP request)
- ❌ **WSGI frameworks** (Flask, Django WSGI): NOT automatically isolated - threads are reused across requests

**Security risk with WSGI:**
```python
# WSGI thread pool scenario
# Request A (Thread 1) → BedrockAgentCoreContext.set_workload_access_token("USER_A_TOKEN")
# Request B (Thread 1) → BedrockAgentCoreContext.get_workload_access_token()
# → Returns "USER_A_TOKEN" ❌ (LEAKED!)
```

This causes:
- Token leakage between requests
- Authorization bypass vulnerabilities
- Data access from one user by another

## Current Status

The AgentCore SDK correctly uses **Starlette (ASGI)**, making it safe. However, there's no documentation warning against using `BedrockAgentCoreContext` in non-ASGI environments.

## Recommendation

Add documentation to explicitly state:

1. **In `src/bedrock_agentcore/runtime/context.py` docstring:**
- BedrockAgentCoreContext is designed for ASGI web frameworks only
- Using with WSGI frameworks creates security vulnerabilities
- ContextVars rely on async task isolation, not thread isolation

2. **In README or main documentation:**
- Note that the SDK requires ASGI (already uses Starlette)
- If users want to integrate with Flask/WSGI, they need alternative approaches (Flask's `g` object, thread-local storage with manual cleanup)

3. **Consider adding a runtime check** (optional):
- Detect if running in WSGI context
- Emit warning about unsafe usage

## References

- BedrockAgentCoreContext implementation: `src/bedrock_agentcore/runtime/context.py:16-21`
- Context initialization in request handler: `src/bedrock_agentcore/runtime/app.py:299-347`
- Python ContextVar documentation: https://docs.python.org/3/library/contextvars.html

## Impact

- **Severity**: High (security vulnerability if misused)
- **Likelihood**: Low (SDK uses Starlette, but users might try to reuse context pattern)
- **Action**: Documentation update to prevent misuse

Beginne mit src/bedrock_agentcore/runtime/context.py:16-21 und überprüfe anschließend die Einrichtung des Request-Kontexts in src/bedrock_agentcore/runtime/app.py:299-347. Dokumentiere, dass BedrockAgentCoreContext für die Verwendung mit ASGI vorgesehen ist, und erläutere das Isolationsrisiko von ContextVar bei WSGI; aktualisiere die README oder die während dieser Überprüfung ermittelte Hauptdokumentationsstelle. Als abgeschlossen gilt die Aufgabe, wenn Benutzer die Anforderung, ausschließlich ASGI zu verwenden, und die daraus resultierende Sicherheitsfolge klar erkennen können.

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