fix(hermes): preserve artifact blocks when converting to native content

[Adityakk9031]

Summary

When exporting or converting transcripts with Block::Artifact to Hermes format (Hermes::from_common), artifact blocks were completely dropped.

In src/harness/hermes.rs, native_content only filtered for Block::Text and Block::Image. Any Block::Artifact was matched by _ => None and discarded. If a message consisted solely of an artifact (or text + artifact), the artifact content was completely omitted from the resulting Hermes message content.

Other harnesses without native artifact storage (such as Pi, Cursor, Fx, Grok, and Codex) retain artifact content using artifact.display_text().

Changes

• Updated native_content in src/harness/hermes.rs:
  • In the string content path: extract artifact.display_text() and append it to the text blocks joined by \n.
  • In the multimodal array path: emit {"type": "text", "text": artifact.display_text()} for any Block::Artifact.
• Added regression test from_common_preserves_artifacts in tests/integration/hermes.rs checking both user and assistant turns.