fix(instances): accept decimals and improve SetInstanceResources error handling

[laolusrael]

Summary

Fixes PUT /api/instances/{name}/resources failing with generic errors and rejecting valid decimal memory/disk values.

Changes

1. Accept decimal values in memory/disk strings

   • parseMemoryString now uses strconv.ParseFloat instead of strconv.ParseInt
   • Allows values like "4.0G", "25.0G", "2.5G" that multipass accepts

2. Capture stderr from multipass commands

   • SetInstanceResources now uses cmd.CombinedOutput() instead of cmd.Output()
   • Returns actual multipass error messages (e.g., "setting must be a positive integer") instead of generic "exit status 1"

3. Skip unchanged values

   • Only calls multipass set for values that actually changed
   • Prevents unnecessary errors when partial updates are sent

Test Results

• go test -tags ci ./... -- Pass
• gofmt -d . -- Clean

Fixes

• Resolves {"error":"invalid_request","message":"invalid memory format"} for "4.0G"
• Resolves {"error":"multipass_error","message":"failed to set CPU: exit status 1"} by surfacing real errors