[BUG] Anthropic streaming wrapper returns wrong object, breaking get_final_message()

Description

The _MessageStreamManager wrapper in openinference-instrumentation-anthropic wraps the wrong stream object when entering the context manager. It wraps anthropic.Stream (low-level iterator) instead of anthropic.lib.streaming._messages.MessageStream (high-level stream with helper methods), causing AttributeError: 'Stream' object has no attribute 'get_final_message'.

Environment

• Package: openinference-instrumentation-anthropic==0.1.20 (latest)
• Anthropic SDK: anthropic==0.71.0
• Python: 3.12
• Context: Using with CrewAI framework

Steps to Reproduce

import os
from anthropic import Anthropic
from opentelemetry import trace
from opentelemetry.sdk.trace import TracerProvider

# Setup tracing
provider = TracerProvider()
trace.set_tracer_provider(provider)

# Instrument Anthropic
from openinference.instrumentation.anthropic import AnthropicInstrumentor
AnthropicInstrumentor().instrument()

# Create client and use streaming
client = Anthropic(api_key=os.getenv("ANTHROPIC_API_KEY"))

with client.messages.stream(
    model="claude-3-5-haiku-20241022",
    max_tokens=50,
    messages=[{"role": "user", "content": "Say hello"}],
) as stream:
    # Consume stream
    for event in stream:
        if hasattr(event, "delta") and hasattr(event.delta, "text"):
            print(event.delta.text, end="", flush=True)

    # Try to get final message - THIS FAILS
    final_message = stream.get_final_message()  # ❌ AttributeError

Expected Behavior

The wrapped stream should have get_final_message() method available, just like the unwrapped MessageStream:

# WITHOUT instrumentation - works ✅
with client.messages.stream(...) as stream:
    # stream is MessageStream
    final_message = stream.get_final_message()  # Works!

Actual Behavior

AttributeError: 'Stream' object has no attribute 'get_final_message'

Root Cause Analysis

The wrapper wraps the wrong object:

with stream_manager as stream:
    print(type(stream))
    # <class 'openinference.instrumentation.anthropic._stream._MessagesStream'>

    print(type(stream.__wrapped__))
    # <class 'anthropic.Stream'>  ❌ WRONG!
    # Should be: <class 'anthropic.lib.streaming._messages.MessageStream'>

    print(hasattr(stream.__wrapped__, 'get_final_message'))
    # False ❌

What's wrapped: anthropic.Stream (low-level iterator)

• Has only: close, response
• Missing: get_final_message(), until_done(), text_stream, etc.

What should be wrapped: anthropic.lib.streaming._messages.MessageStream

• Has all helper methods including get_final_message()

Impact

This breaks any code that uses get_final_message() with Anthropic streaming, including:

• CrewAI framework's Anthropic integration
• Any application using the standard Anthropic streaming pattern from their docs
• Token usage tracking that depends on final message metadata

Proposed Fix

The __enter__ method of _MessageStreamManager should return the actual MessageStream object, not the underlying Stream iterator.

Additional Context

• This appears to be a regression or oversight in the streaming support
• Issue [anthropic] add support for streaming #986 mentioned adding streaming support, but get_final_message() delegation seems incomplete
• ObjectProxy from wrapt should delegate methods, but it's wrapping the wrong object