feat: add custom max_tokens for [summary_pdf, task_planner] into DocPixieConfig

[liemkg1234]

Generate document summary using provider

summary = await self.provider.process_multimodal_messages(
messages=messages,
max_tokens=400,
temperature=0.3
)

Generate initial plan

result = await self.provider.process_text_messages(
messages=messages,
max_tokens=8192,
temperature=0.3
)