Fix agent diff hanging when we "Keep All" changes for removed file

[max-frai]

Closes #32158

Release Notes:

• Fix agent diff hanging when we "Keep All" changes for removed file

When agent removes some file, we still can see this change in the list of changed files. There we can click Review on removed file. Clicking "Keep All" hangs forever with this (simplified) log:

ERROR [zed::reliability] Suspected hang on main thread:
...
5. alloc::collections::btree::map::Keys<gpui::app::entity_map::Entity<language::buffer::Buffer>, gpui::app::entity_map::Entity<buffer_diff::BufferDiff>>  
   ↳ core::iter::Iterator::try_fold
6. core::iter::adapters::cycle::Cycle<alloc::collections::btree::map::Keys<…>>  
   ↳ core::iter::Iterator::find
7. agent_diff::AgentDiff::review_in_active_editor
8. gpui::app::AppContext::update_entity<agent_diff::AgentDiff, Option<executor::Task<…>>>

Zed tried to use cycle iterator to find buffer for the removed file, but it doesn't exist. In this case we just take next available buffer.

[cla-bot]

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[max-frai]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[zed-industries-bot]

	Warnings
⚠️	This PR is missing release notes. Please add a "Release Notes" section that describes the change: Release Notes: - Added/Fixed/Improved ... If your change is not user-facing, you can use "N/A" for the entry: Release Notes: - N/A

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[filipwiech]

Might also fix #32492. 👍

[notpeter]

Apologies for not getting a chance to review this sooner, but I believe this has been fixed. If you are still seeing this can you comment with more details in the original issues and we can reopen there?

Thanks!