perf: reduce root import overhead

[wilyan09007]

What

Lazily resolves heavy root re-exports from anthropic.__init__ so import anthropic no longer eagerly imports generated types, tools, Bedrock, Vertex, streaming, credentials, schema-transform modules, or the base Anthropic client stack.

The root package now defers _client, _types, _models, _base_client, _response, _constants, _exceptions, httpx, and pydantic until the corresponding exported attribute is first used. It also avoids importing anthropic.types from _exceptions.py at runtime just to satisfy the ErrorType annotation.

Why

Fixes #1211.

The issue reports slow root imports caused by eager type/client/tool imports. Deferring those modules keeps import anthropic lightweight while preserving anthropic.Anthropic, from anthropic import Anthropic, and the existing root re-export surface.

Verification / How to test

• uv run pytest tests\test_imports.py tests\lib\test_vertex.py tests\lib\tools\test_functions.py -q
• uv run ruff check src\anthropic\__init__.py src\anthropic\_exceptions.py tests\test_imports.py
• uv run ruff format --check src\anthropic\__init__.py src\anthropic\_exceptions.py tests\test_imports.py
• uv run pyright src\anthropic\__init__.py src\anthropic\_exceptions.py tests\test_imports.py
• git diff --check HEAD~1..HEAD
• 10 subprocess timing runs for import anthropic: min 19.1ms, median 19.9ms, max 26.3ms
• Import-state probe confirms import anthropic does not load httpx, pydantic, anthropic._client, or anthropic._types.

I also ran uv run pytest tests\test_client.py tests\test_response.py tests\test_legacy_response.py -q -n0; it reached 204 passed and failed on the existing Windows-local memory-leak sentinel and proxy-environment mount assertions, not on lazy import behavior.