fix: show and write JSON file contents sent as a JSON value

[YauhenBichel]

Description

Fixes #13362 and #13363.

Some models (Qwen in particular) send the contents of a new .json file as a JSON value instead of a string. Because the tool call arguments are parsed deeply:

• the tool call preview showed [object Object] (Agent: creating a JSON file shows [object Object] instead of its contents #13362);
• create_new_file wrote the file on one line, through getStringArg's compact JSON.stringify (create_new_file writes a JSON file on one line when the model sends its contents as a JSON value #13363).

A new stringifyFileContents() in core/tools/parseArgs.ts turns such a value into indented JSON. createNewFileImpl uses it when contents is an object or array, and the GUI preview uses it too, so the preview shows exactly what will be written. getStringArg is unchanged: compact JSON stays right for other string arguments, and its existing tests still pass.

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Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created (none needed)
• The relevant tests, if any, have been updated or created

Screen recording or screenshot

Not included; the GUI test below renders the preview and checks its text.

Tests

• gui/src/pages/gui/ToolCallDiv/CreateFile.test.tsx (new): the preview of a create_new_file call with object contents shows indented JSON, not [object Object]; string contents are unchanged. Fails before the fix.
• core/tools/implementations/createNewFile.vitest.ts (new): object contents are written as indented JSON; string contents unchanged. Fails before the fix.
• core/tools/parseArgs.vitest.ts: tests for stringifyFileContents.

npx vitest run tools/parseArgs.vitest.ts tools/implementations/createNewFile.vitest.ts in core (54 passed) and npx vitest run src/pages/gui/ToolCallDiv/ in gui (22 passed).

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