feat: wrappers for async messages methods

[eternalcuriouslearner]

Description

This change adds sync and async wrappers for Anthropic message streams, keeps telemetry finalization in the wrapper layer, and adds tests for wrapper behavior. It does not wire the wrappers into instrumentation yet.

Fixes #3949 partly

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• python3 -m pytest -q instrumentation-genai/opentelemetry-instrumentation-anthropic/tests/test_async_wrappers.py
• python3 -m pytest -q instrumentation-genai/opentelemetry-instrumentation-anthropic/tests/test_sync_messages.py
• python3 -m pytest -q instrumentation-genai/opentelemetry-instrumentation-anthropic/tests/test_instrumentor.py

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated