[CHORE] Remove graduated experimental shims (steering, tools.ToolProvider, hooks aliases)

[yonib05]

Problem Statement

Several features have already graduated out of strands.experimental into the production namespace, but the experimental locations still exist as DeprecationWarning shims that re-export from production. Per the SDK graduation lifecycle (versioning-and-support.mdx: X.Y = graduate + deprecate; X.Y+1 = experimental version removed), these shims are now eligible for removal.

They were introduced on 2026-05-26 in the monorepo convergence (#2317), so they have been deprecated for multiple minor releases.

Shims to remove (strands-py/src/strands/experimental/)

• steering/ — entire subtree is a DeprecationWarning shim re-exporting from strands.vended_plugins.steering. Files: __init__.py, core/{__init__,action,context,handler}.py, context_providers/{__init__,ledger_provider}.py, handlers/__init__.py, handlers/llm/{__init__,llm_handler,mappers}.py.
• tools/__init__.py — ToolProvider moved to strands.tools; __getattr__ warns and re-exports. __all__ is already empty.
• hooks/multiagent/ (__init__.py, events.py) — deprecated, re-exports from strands.hooks.multiagent.
• hooks/__init__.py non-bidi aliases — BeforeToolInvocationEvent, AfterToolInvocationEvent, BeforeModelInvocationEvent, AfterModelInvocationEvent are deprecated aliases resolved via __getattr__. Keep the live Bidi* events (still experimental, see [FEATURE][EPIC] Bidirectional Streaming - Move out of Experimental #1722); remove only the graduated aliases from __all__.
• bidi/tools/stop_conversation.py — deprecated tool (.. deprecated::, warns on use); use strands_tools.stop. Note this one is gated on the bidi graduation ([FEATURE][EPIC] Bidirectional Streaming - Move out of Experimental #1722) since it lives under bidi/.

Also remove

• The corresponding alias-only tests that exist purely to assert the deprecation warnings fire:
  • tests/strands/experimental/steering/test_steering_aliases.py
  • tests/strands/experimental/tools/test_tool_provider_alias.py
  • tests/strands/experimental/hooks/test_hook_aliases.py (non-bidi alias cases only)

Out of scope

• Live experimental features that have not graduated: checkpoint ([FEATURE] Durable Execution via Provider-Agnostic Checkpoints (Epic) #2510), agent_config ([FEATURE] Graduate Agent Config out of Experimental #1625), and the full bidi suite + Bidi* hook events ([FEATURE][EPIC] Bidirectional Streaming - Move out of Experimental #1722).

Acceptance criteria

• Importing a removed symbol from its old experimental.* path raises ImportError/AttributeError (not a silent success).
• No production code imports from the removed shim paths (rg clean).
• CHANGELOG / migration note points users at the production import paths.