fix: #3319 preserve nested handoff history content

[Aphroq]

Summary

• Preserve multiline message content when flattening an already nested handoff-history summary.
• Preserve structured role content by serializing those summary entries as JSON and parsing them back before the next history mapper runs.
• Keep the existing readable role: content format for simple string messages, and keep legacy summary parsing for older saved histories.

Test plan

• bash .agents/skills/code-change-verification/scripts/run.sh

Issue number

Closes #3319

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass

[seratch]

Thanks for the fix. I agree this is a real scenario: chained nest_handoff_history can receive multiline user content or structured/multimodal content, and preserving that transcript shape is important.

Before merging, could you make one small follow-up change? The new record splitter still treats a continuation line like 2. not a new record as a new summary item, so multiline string content can still be truncated when a later line happens to look like a numbered summary record.

Could we serialize role items with string content that contains newlines using the JSON record format as well? That should remove the ambiguity for newly generated summaries while keeping the legacy parser for older saved histories. Please also add a regression test for a multiline string where the second line starts with something like 2. not a new record.

[Aphroq]

Thanks for the review.

I changed the formatter so role items with string content that contains \n or \r now use the JSON record format too. Simple one-line string messages still keep the readable role: content format, and the legacy parser still handles older saved summaries.

I also updated the multiline regression test to use first line\n2. not a new record, so the new summary path no longer depends on the record splitter guessing whether that second line is content or a new item.

I checked the adjacent cases as well: structured/multimodal role content still round-trips through JSON, legacy multiline summaries still parse, role+name formatting is preserved for simple strings, non-role items still serialize as JSON, provider_data is still stripped, and the existing custom-wrapper/unserializable fallback tests still pass.