fix(runtime): update shell session wire protocol

[priyasingh2111]

Issue #, if available:

Description of changes:
Updates the shell session wire protocol to simplify the connection handshake and remove deprecated features:

• Remove metadata frame handshake: Connection is ready immediately after WebSocket 101 upgrade — shellId is now read from the response header instead of waiting for a STATUS frame. Eliminates the blocking `_read_metadata_frame()` step and the `_pending_frames` queue.
• Remove `encode_close()` frame: `close()` no longer sends a CLOSE frame — the shell detaches and stays alive for the reconnect window.
• Simplify `on_reconnect` callback: No longer passes a `reconnected: bool` argument — callback takes no parameters.
• Remove `reconnected` and `bytes_dropped` attributes: No longer needed without the metadata frame flow.

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[github-actions]

⚠️ Breaking Change Warning

Found 3 potential breaking change(s) in this PR:

�[1msrc/bedrock_agentcore/runtime/shell/protocol.py�[0m:0: ShellFramer.encode_close: �[33mPublic object was removed�[39m
�[1msrc/bedrock_agentcore/runtime/shell/session.py�[0m:0: ShellSession.reconnected: �[33mPublic object was removed�[39m
�[1msrc/bedrock_agentcore/runtime/shell/session.py�[0m:0: ShellSession.bytes_dropped: �[33mPublic object was removed�[39m

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Note: This is an automated static analysis check. Some flagged changes may be intentional.
Please confirm each item is expected and, if so, add a migration note to CHANGELOG.md.

[agentcore-devx-automation]

Claude Security Review: no high-confidence findings. (run)

[aidandaly24]

Could we update the open_shell() docstring in src/bedrock_agentcore/runtime/agent_core_runtime_client.py? It still shows on_reconnect(reconnected: bool) and says that exiting the context sends a CLOSE frame. The callback now takes no arguments, and exiting the context now detaches the shell.

I can also make this change after this is merged, but will reapprove if you update. Thanks.