[codex] Recover streaming tool turn text

[gold-silver-copper]

Recover assistant text from the prior streaming tool-call turn when the provider's terminal follow-up turn is empty.

Validation:

• cargo fmt --check
• cargo test -p rig-core agent::prompt_request::streaming::tests::
• cargo test --test anthropic prompt_loop_preserves_pre_tool_text_when_terminal_followup_is_empty