Avoid 'dancing consoles' at startup

[jmcphers]

This PR makes Positron startup much calmer when multiple affiliated runtimes exist. The changes are pretty simple:

• When firing events that indicate that a runtime will start, note whether we plan to activate the runtime at startup, and use that in the Console to decide whether to flip to the "Preparing" state for the runtime.
• Don't make every session that starts the foreground session; only do this if the session is marked for activation.
• When selectLanguageRuntime is called during startup, do not allow it to override the runtime selection process that happens at startup. (The Python extension does this.)

With these changes, you will also notice fewer unwanted Pythons starting in R environments. Since these changes make it possible for the Python console to go completely unused (never focused) in a Positron session, the dormant logic that switches off unused languages can come back into play.

There is a big caveat here, however, which is that the Python extension is still aggressively starting Python in workspaces that don't want or need it. Depending on the timing, this can negate much of the benefits of the above changes; once startup is complete from Positron's perspective, it allows the Python extension to start a new Python if it wants to, and this will focus the new Python session and establish an affiliation. I didn't try to tackle this problem since it is scheduled for work soon in #12116.

Addresses #10929.

Release Notes

New Features

• N/A

Bug Fixes

• Reduce flickering/switching at startup between languages in the Console for projects that use R and Python (Console: Return of the Dancing Consoles (flipping between R/Python at boot) #10929)

QA Notes

This change specifically addresses startup issues for workspaces that have affiliated runtimes (that is, we know ahead of time which to start). If you observe problematic behavior, use the Runtime Startup Diagnostics report. If you see this, you should have a "calm" startup:

Runtime	Name	Mode	State	Start Reason	Created
Python 3.12.10 (Pyenv)	Python 3.12.10 (Pyenv)	console	idle	Affiliated Python runtime for workspace	11:23:14 AM
R 4.5.0	R 4.5.0	console	idle	Affiliated R runtime for workspace	11:23:14 AM

If you see Python starting due to "extension requested" activation, then you may still experience some dancing at startup.

Test tags: @:console @:sessions

[github-actions]

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

^{readme  valid tags}

[juliasilge]

This improves the experience when you have multiple affiliated runtimes A LOT. 🙌

With these changes and 3 affiliated runtimes, I don't see them grabbing focus from each other as they come online.