Discriminate log messages by session ID

[lionel-]

Branched from #10933
Progress towards #10121

This PR:

• Makes the kernel logs less confusing when multiple sessions are running concurrently. This should help making sense of CI logs for Ark - Extensions tests unstable due to LSP session issues #10121.

• Reduces the number of output channels, making it easier for users to find and report the relevant channels.

Approach:

• There is now only one Kernel channel, one Supervisor channel, and one Language Server channel for all sessions, console or notebook.
• All log messages are prefixed with the session ID (which includes a notebook particle for notebooks).
• The language server does not need the session prefix because there is only one active at a time, so the session start message, which includes the session ID, is sufficient to disambiguate sessions.

This is going to make logs more busy but will greatly improve disambiguation in debugging session. The "Filter" feature of output channels can also be used to filter messages for a particular session, if needed.

The alternative approach I first considered was to include the session ID in output channel names. The issue is that this leads to an explosion of output channels (3 by session ID) that are never cleaned up.

If we go forward with this, I'll send a companion PR to update https://positron.posit.co/troubleshooting.html#python-and-r-logs.

Here is how this looks:

Screen.Recording.2025-12-05.at.10.22.27.mov

Screen.Recording.2025-12-05.at.10.23.01.mov

Screen.Recording.2025-12-05.at.10.23.41.mov

Release Notes

New Features

• Log output channels have been consolidated. There is now a single Kernel, Supervisor, and Language Server channel per language.

Bug Fixes

• N/A

QA Notes

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical

^{readme  valid tags}

[jmcphers]

Nit: this makes the output channel open very early and it'll be around even for Python-only users. Not a big deal.