feat(rehydrate): add max_history_bytes and max_history_items caps

[crstrn13]

Summary

Closes #66.

• Add max_history_bytes (default 2 MiB) and max_history_items (default unlimited) config fields to the openai_responses_rehydrate filter
• Enforce limits before stored history is spliced into ResponsesState, preventing unbounded proxy memory growth
• Reject oversized history with 413 and an actionable error message ("compact or shorten the conversation before continuing")
• Introduce a RehydrationSource trait so build_state is generic over ResponseRecord and ConversationRecord — limits are enforced uniformly on both the previous_response_id and conversation rehydration paths
• Restructure rehydrate into a clean dispatcher with two symmetric methods: rehydrate_from_response and rehydrate_from_conversation
• Fix fail-open bug: serialization failure in byte-size measurement now falls back to usize::MAX (reject) instead of 0 (allow)

Config

- filter: openai_responses_rehydrate
  max_history_bytes: 2097152  # 2 MiB (default)
  max_history_items: ~        # unlimited (default)

Both fields reject zero values at config load time.

Test plan

• 55 unit tests pass (cargo test -p praxis-ai-apis -- rehydrate)
• Config validation: rejects zero max_history_bytes, zero max_history_items, unknown fields
• Response path: rejects byte limit, item limit, exact boundary, streaming 413 format
• Conversation path: rejects byte limit, item limit, within-limits passthrough
• make lint clean (clippy + nightly fmt + filter docs)

[franciscojavierarceo]

One blocking memory-boundary concern is noted inline.

[franciscojavierarceo]

[P1] Enforce the limits before cloning the stored history. source.stored_messages() materializes the entire Vec here, while ConversationRecord/ResponseRecord have already been fully deserialized by get_conversation/get_response; enforce_history_limits therefore runs only after the potentially unbounded allocation has happened. A large accumulated conversation can still cause a substantial memory spike before returning 413, so max_history_bytes does not provide the advertised memory bound. Please enforce the item/byte limits on the borrowed stored JSON (or fetch a bounded representation) before cloning it into the request state.