acp_thread: Preserve tool presentation on invalid content

[benbrandt]

Summary

Stacked on #64765, using benbrandt/acp-message-updates as the base rather than the keyed-message PR.

• Prepare all incoming tool content before changing presentation fields or shared display entities. A missing terminal or failed content conversion no longer leaves a partially updated title, metadata, content list, or terminal command label.
• Share preparation between creation and updates, filtering unsupported entries before positional reconciliation without constructing a disposable second set of views.
• Preserve raw-input Markdown identity across snapshots and skip rendering unchanged input.
• Reuse unchanged diffs without constructing discarded entities, and include the path in the reuse check so identical text at a different path does not retain the old file.

Compatibility boundary

Reported status remains authoritative even when content conversion fails. Nonterminal updates preserve an open permission request; terminal status still settles it and resumes sleep prevention as before. The existing failed-tool update/upsert lifecycle tests remain unchanged.

This makes presentation updates atomic, not the whole tool update. Existing error propagation, v1 omission semantics, raw-output fallback, and specialized native diff/terminal updates remain intact. It does not add v2 tri-state APIs, a canonical wire-state store, or protocol negotiation.

Validation

• All 176 acp_thread tests pass.
• Three new regressions pass across 100 scheduler seeds: failed presentation with authoritative status, display identity and changed diff paths, and creation retry/terminal-label isolation.
• All 461 agent_ui tests pass with --test-threads=1.
• All 41 agent_servers tests and five native replay tests pass.
• ./script/clippy passes for acp_thread, agent_ui, agent, agent_servers, and acp_tools.
• Formatting and diff checks pass; independent review found no blocking findings.

Suggested .rules additions

For the ACP-thread crate, proposed for a separate rules change:

Do not roll back a reported terminal tool status when output content cannot be converted. Permission settlement and sleep prevention depend on status application; validate presentation changes separately and preserve the failed-tool update/upsert lifecycle tests.

Release Notes:

• Fixed partial tool-call display updates when content references an unavailable terminal.