Fix ClearToolResults corrupting typed ToolReturnPart subclasses (ToolSearchReturnPart)

[dsfaccini]

This pull request was posted by Claude Code using claude-opus-4-8 on behalf of David.

Summary

ClearToolResults blanks old tool results in place. rebuild_with_cleared selected
them with isinstance(part, ToolReturnPart), which also matches typed subclasses such
as ToolSearchReturnPart (core's search_tools return). dataclasses.replace preserves
the concrete class, so blanking the content produced a ToolSearchReturnPart whose
structured TypedDict content was now a plain placeholder string.

Core's parse_discovered_tools trusts that content shape (docstring: "No defensive
isinstance walks needed") and re-reads it on every request via content['discovered_tools'].
Once the corrupted history is written back into run state, the run's next request raises
TypeError: string indices must be integers.

The fix narrows the request-side clearing predicate to an exact type check
(type(part) is ToolReturnPart), so typed subclasses are skipped rather than blanked.
They carry discovery state core re-reads each request and reclaim little, so skipping
them is both correct and near-free. The response-side ToolCallPart args-clearing branch
is unchanged (its placeholder is type-valid there). A code comment guards against a future
revert to isinstance.

Linked Issue

Fixes #380

Checklist

• Linked issue exists and is referenced above
• Tests added/updated for new behavior
• make lint && make typecheck && make test passes locally (don't stress about CI -- we'll help)
• No changes to pyproject.toml or uv.lock (dependency changes require a separate issue)
• Docstrings use single backticks (not RST double backticks)

[macroscopeapp]

Approvability

Verdict: Approved

Straightforward bug fix with clear intent and minimal scope. The single-line change narrows a type check from isinstance to exact type match, preventing corruption of typed subclasses. Well-documented with explanatory comment and regression test.

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