fix: add support for tool:post ephemeral injections

Added _pending_ephemeral_injections instance variable to store ephemeral
injections when tool:post fires, then apply them on the next iteration
when building messages for the LLM request.

This enables hooks that fire on tool:post to inject context that will
be included in the next LLM request.

Also removed unused imports (Optional, PLAN_START, PLAN_END).

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Co-Authored-By: Amplifier <240397093+microsoft-amplifier@users.noreply.github.com>

Author: bkrabach