Fixes for send_message_with_attachment, changing model mid-task, and task pagination (329, 380, 381)

[makiroll1125]

What

• Redact the API key from [LLM FACTORY] log lines instead of logging it in plaintext
• Stop the task list from jittering/rapid-refetching when scrolling to the top or bottom of the Tasks page or Chat sidebar
• Keep in-progress tasks' chat history and caches intact when only the model (not the provider) is changed mid-task
• Keep active tasks pinned at the top and load them all up front, so they no longer "pop in" and reorder while scrolling
• Stop the agent from silently waiting 3 hours for a reply when sending a message with an attachment actually failed

Why

A batch of small bug fixes, mostly UI glitches and edge cases that caused silent failures or confusing behavior, plus one security fix to stop an API key from leaking into logs.

Closes #329 , #380 , and #381

How to test

• API key redaction: Configure a BytePlus provider and start the app; check the logs for the [LLM FACTORY] line and confirm the API key shows as <redacted> instead of the real value.
• Task list jitter: Open the Tasks page (or the Chat page's task sidebar) with enough history to scroll, and scroll rapidly to the very top and very bottom. Confirm the list no longer jitters or flickers.
• Model-switch mid-task: Start a long-running task, then go to Settings → Models and change only the model (same provider) while it's running. Confirm the task continues without losing context, and that saving Settings with no changes doesn't disturb running tasks either.
• Task pagination pop-in: Seed a mix of active and completed tasks where some active tasks are older than the most recent completed ones. Confirm all active tasks show up immediately at the top with no scroll needed, and that scrolling to load older tasks never causes tasks to move or reorder.
• Attachment send failure: Trigger send_message_with_attachment with a file path that doesn't exist. Confirm the action reports an error and the agent does not go silent waiting for a reply.