Emit status events from dev-session

[isaacroldan]

WHY are these changes introduced?

Improve DevSession feedback by showing the current status in the dynamic section of the terminal log footer.

WHAT is this pull request doing?

• Adds specific error cause for app validation errors
• Introduces new status message handlers for different dev session states (build errors, ready state, loading, etc.)
• Ensures proper status updates when extensions are affected by changes

How to test your changes?

1. Run dev command with dev-sessions enabled.
2. Verify status messages appear correctly for:
   • Initial loading state
   • Build errors (make a bad change in a extension code)
   • Validation errors (make a bad change in a toml file)
   • Successful updates
   • Ready state
3. Confirm status message automatically resets to "Ready" 2 seconds after an update

Measuring impact

How do we know this change was effective? Please choose one:

• n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes

[isaacroldan]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Emit status events from dev-session #5374 👈 (View in Graphite)
• Show a dynamic dev session status message in the UI #5356
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	75.5% (+0% 🔼)	9067/12009
🟡	Branches	70.78% (+0.01% 🔼)	4420/6245
🟡	Functions	75.21%	2379/3163
🟡	Lines	76.01% (+0% 🔼)	8563/11265

Show files with reduced coverage 🔻

St.❔	File	Statements	Branches	Functions	Lines
🟢	... / ConcurrentOutput.tsx	98.36% (-1.64% 🔻)	88% (-4% 🔻)	100%	98.33% (-1.67% 🔻)

Test suite run success

2044 tests passing in 913 suites.

Report generated by 🧪jest coverage report action from 60e20d6