fix(codex): preserve interleaved text and tool call block ordering on export

[Adityakk9031]

When converting transcripts from Common to Codex via Codex::from_common, push_message_lines buffered all text and artifact blocks across the entire message while pushing Thinking, ToolUse, and ToolResult items immediately. At the end of the turn, the buffered text was flushed as a single response item.

In assistant turns where text precedes a tool call (for example [Block::Text("Checking directory..."), Block::ToolUse { ... }]), this inverted the order in the exported rollout JSONL: the tool call was emitted first, followed by the explanation text. In turns with text before and after a tool call, both text segments were lumped together after the tool call.

This change flushes any accumulated text content before emitting reasoning, function_call, or function_call_output records, preserving the chronological sequence across interleaved turns.

Testing

• Added from_common_preserves_interleaved_block_ordering in tests/integration/codex.rs verifying that text before and after a tool call maintains exact line ordering in the emitted rollout.
• Verified round-trip fixpoint test codec_fixpoint_through_common_loses_nothing continues to pass.
• cargo test -p txcript --no-default-features --features opencode,hermes,search --test integration codex passed.
• cargo clippy -p txcript --no-default-features --features opencode,hermes,search --all-targets -- -D warnings clean.