Closes#608. Make Message.content officially Optional (str | None) with a Message.text accessor that treats None as empty, and count tool-call IDs/names/arguments, tool-result IDs, and reasoning/thinking metadata in estimate_prompt_tokens — all are replayed into later prompt turns, so they belong in the estimate. Extends estimator and message-type regression tests.
Closes#607. Assistant tool-call turns can carry content=None, which crashed token estimation (len(m.content)) and think-tag stripping. Normalize with 'content or ""', route native OpenHands truncation through the shared estimate_prompt_tokens, and add tests for the estimator, the truncation helper, and an end-to-end tool-call run with None content.
