feat: return 409 if agent already booted

[iFergal]

I think 409 is more appropriate here, and gives it some separation from the various other 400 responses from this endpoint without relying on text responses which are more subject to change.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.71%. Comparing base (18d3ad7) to head (7cf15df).
Report is 23 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #302      +/-   ##
==========================================
+ Coverage   93.06%   93.71%   +0.65%     
==========================================
  Files          36       36              
  Lines        7121     7941     +820     
==========================================
+ Hits         6627     7442     +815     
- Misses        494      499       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[2byrds]

Seems reasonable "The HTTP 409 Conflict client error response status code indicates a request conflict with the current state of the target resource."