fix file_edit dropping an explicit empty new_string

[RaajeevChandran]

Resolves #3031

• file_edit rejected new_string: "" with "old_string given without new_string", even though "" is the documented way to delete a match
• Root cause: SchemaValidator.dropEmptyOptionalStrings runs during preflight and removes optional string arguments that are empty. new_string is optional because the edits batch form exists, so it was removed before FileEditTool ever saw it
• PR Harden file-edit tools for document drafting (Raptor + grok-4.3 proof) #2914 added the empty new_string handling inside FileEditTool, but its tests called the tool directly and never went through preflight, so the bug was never caught
• Added preservedEmptyStringArguments to OsaurusTool (default empty). ToolRegistry.preflight passes it into coerceArguments(preservingEmpty:) and those arguments keep their empty value
• Host file_edit keeps new_string. Sandbox file_write keeps new_string and content, which also fixes the documented content: "" truncate to an empty file
• The wire schema is unchanged, so tool definitions sent to providers and the prompt prefix stay the same
• Tests: a coercion unit test for preservingEmpty, plus a FileEditBatchTests case that runs the reporter's exact repro through preflight and then the tool

Changes

• Behavior change
• UI change (screenshots below)
• Refactor / chore
• Tests
• Docs

Checklist

• I have read CONTRIBUTING.md
• I added/updated tests where reasonable
• I updated docs/README as needed
• I verified build on macOS with Xcode 16.4+

[jjang-ai]

This merged after the 0.25.20 tag was created, so it is not included in the 0.25.20 binary. The release workflow read the mutable draft later and accidentally listed/marked it released. I removed the incorrect public release note and restored pending release; #3049 corrects the same line in the appcast. Your fix remains merged for the next release.