fix(providers): handle empty assistant replies and missing tool results for strict OpenAI endpoints

[hw0rld]

What

Fixes two HTTP 400 crashes when running against strict OpenAI-compatible
endpoints (DeepSeek). Both crashes killed live engagements and made resumed
sessions unusable.

Bug 1: empty assistant messages

When the target returns a reasoning-only reply (content empty, CoT present),
query_target / continue_target persisted an assistant message with empty
content into the target thread. DeepSeek rejects the next request with:

Invalid assistant message: content or tool_calls must be set

Fix:

• tools/target.py _persist_thread: an empty reply no longer appends an empty
  assistant message to the thread.
• tools/target.py _continue_target: an empty reply folds the recovered CoT
  into the message body, or uses a small marker when even CoT is missing.
• providers/openai_provider.py _messages_to_wire: defense-in-depth — skip
  assistant messages with neither text nor tool calls (also covers resumed
  sessions that already contain poisoned history).

Bug 2: unfinished tool-call rounds on resume

A session that crashed mid-tool-execution can end with an assistant message
carrying tool_calls but no recorded tool results. Resuming and sending the
next turn makes DeepSeek reject the request with:

An assistant message with 'tool_calls' must be followed by tool messages
responding to each 'tool_call_id'. (insufficient tool messages following
tool_calls message)

Fix:

• providers/openai_provider.py: new _complete_missing_tool_results() inserts
  placeholder tool results for any missing tool_call_id when serializing to
  the wire, so recovered sessions stay valid.

Verification

• Real DeepSeek API: the previously-failing resumed-session request now returns
  200 OK.
• pytest tests -k "target or continue or provider or session": 223 passed.
• The two unrelated failures (test_tui_autocomplete resume picker,
  test_tg5_harden missing hypothesis) fail identically on main before this
  change.

Impact

Token cost is negligible (placeholders are a few tokens; empty replies now carry
the already-recovered CoT instead of nothing). No behavior change for healthy
threads.