Support stringified edit tool inputs

[currybab]

Summary

Agent edit tool calls that double-encode mode or edits now deserialize successfully instead of failing before an edit session starts. This backports the relevant behavior from zed-industries/zed#55500 while keeping the change scoped to Superzent's streaming edit file tool.

• Adds a local serde helper that accepts either native JSON values or JSON strings containing those values.
• Applies the helper to final edit-tool input and streaming partial input for mode and edits.
• Adds focused coverage for native values, double-encoded values, omitted optionals, and explicit null optionals.

Testing

• cargo test -p agent deserializes --lib
• cargo test -p agent test_streaming_edit_file_tool_fields_out_of_order_in_edit_mode --lib
• ./script/clippy -p agent

Notes

• Browser testing was not applicable to this Rust agent-tool parsing change. The pipeline browser step could not run because agent-browser is not installed locally.
• Related upstream PRs: edit tool: Support stringified mode parameter (#55498) (cherry-pick to stable) zed-industries/zed#55500 and edit tool: Support stringified mode parameter zed-industries/zed#55498

---

Release Notes:

• Fixed agent edit failures when edit-tool input fields are sent as stringified JSON.