fix(googleai): escape raw newlines in stream response to prevent json decode error

[Phuocminh94]

Description

When using the Gemini API, the streaming response contains raw, unescaped newlines within the "text" field. This breaks the JSON structure during streaming, causing vim.json.decode to throw a fatal runtime error: Expected object key string but found T_OBJ_END.

Solution

This PR sanitizes the line by escaping raw newlines (\n) and carriage returns (\r) before feeding it into the decoder. Additionally, it wraps the decode process in a pcall to ensure plugin stability even if the stream contains other unexpected formatting anomalies.

Error Log

Lua callback:
.../gp.nvim/lua/gp/dispatcher.lua:323: Expected object key string but found T_OBJ_END at character 25
stack traceback:
	[C]: in function 'decode'
	.../gp.nvim/lua/gp/dispatcher.lua:323: in function 'process_lines'