fix: Report a distinct User-Agent for the async client

[jsonbailey]

Overview

The async client (AsyncLDClient) shipped reusing the synchronous PythonClient/<version> User-Agent, so LaunchDarkly cannot distinguish async traffic from sync. Because sync and async ship in the same package, the User-Agent token is the signal that tells them apart.

This adds a distinct async token PythonAsyncClient and threads it through every async request path.

Changes

• impl/http.py: add SYNC_USER_AGENT / ASYNC_USER_AGENT constants; _base_headers(config, user_agent=SYNC_USER_AGENT).
• impl/util.py: _headers(config, user_agent=SYNC_USER_AGENT) passes the token through.
• Async request sites now pass ASYNC_USER_AGENT:
  • impl/aio/transport.py — AsyncSSEFactory streaming headers
  • impl/datasource/async_feature_requester.py
  • impl/datasourcev2/async_polling.py
  • impl/events/async_event_processor.py
• .sdk_metadata.json: register the token on the existing python-server-sdk entry → userAgents: ["PythonClient", "PythonAsyncClient"].

Sync behavior is unchanged (the default token is still PythonClient).

Design note

SDK-2658 originally proposed a separate python-server-sdk-async sdk-meta registry entry. We chose to keep a single entry and add the async token to its userAgents array instead, because async has no independent package and is not a separately-tracked feature.

The analytics/event SDK name python-server-sdk-async already ships (async_client.py); billing confirmed it does not key off the User-Agent, so this change is safe and additive.

Tests

New testing/impl/test_user_agent.py locks the exact wire tokens for both sync and async via _base_headers / _headers. mypy / isort / pycodestyle clean; touched-component suites pass.

Implements SDK-2658.

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Note

Overview
Async AsyncLDClient traffic previously used the same PythonClient/<version> User-Agent as the sync client, so LaunchDarkly could not tell them apart in one package. This PR introduces PythonAsyncClient and wires it through shared header helpers while keeping sync defaults unchanged.

SYNC_USER_AGENT / ASYNC_USER_AGENT are defined in impl/http.py; _base_headers and _headers take an optional user_agent (default PythonClient). Async paths pass ASYNC_USER_AGENT: SSE in impl/aio/transport.py, FDv1 polling in async_feature_requester.py, FDv2 polling in async_polling.py, and event posts in async_event_processor.py.

.sdk_metadata.json adds PythonAsyncClient to the existing python-server-sdk userAgents list. testing/impl/test_user_agent.py asserts the exact wire User-Agent strings for sync and async.

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