chore: Deduplicate shared sync/async evaluator target checking

[jsonbailey]

Overview

Behavior-preserving cleanup extracted from the async SDK implementation branch (epic SDK-60). Touches only files already on main; independent of the async-client / FDv2 work.

Track Internally: SDK-2871

Changes

Evaluator dedup (core):

• ldclient/impl/evaluator_common.py — add a shared check_targets() free function holding the context/user target-matching logic.
• ldclient/impl/evaluator.py — remove the private _check_targets method; route to the shared check_targets().
• ldclient/impl/async_evaluator.py — same: remove the duplicated _check_targets method and route to the shared function.

The sync and async evaluators previously carried byte-identical copies of the target-matching logic. This collapses them into one implementation.

Async test cleanup:

• ldclient/testing/impl/test_async_evaluator.py, test_async_big_segments.py, test_async_flag_tracker.py, test_async_hooks.py, integrations/test_async_redis.py — drop redundant comment-banner section headers so the async test modules match the sync test style. No test logic changes.

Verification

• uv run pytest on the evaluator and async component test files: all pass (async redis suite verified green against a local redis; it connects to a live server as in CI).
• uv run pycodestyle, uv run isort --check --atomic, uv run mypy clean on all changed files.

No CHANGELOG or version changes.

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Note

Overview
Target matching for user and context targets is consolidated into a shared check_targets() in evaluator_common.py. Both Evaluator and AsyncEvaluator now call that function instead of maintaining identical private _check_targets methods.

Async test modules lose decorative section-header comment banners (and one hook-test helper docstring is clarified); no test behavior changes.

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[joker23]

nit comments only