Avoid fatal panic on stale config-retainer project paths in project rebuild flow

[Copilot]

A server crash was occurring in markProjectsAffectedByConfigChanges when config-change bookkeeping referenced a configured project path that no longer existed in the builder map. This made snapshot updates crash instead of continuing through a recoverable stale-state condition.

• Crash condition

  • markProjectsAffectedByConfigChanges assumed every entry in configChangeResult.affectedProjects was present in configuredProjects.
  • Stale entries (e.g. project removed, retainer not yet cleaned) triggered a fatal panic.

• Behavior change

  • Replace panic-on-missing-project with a guarded skip:
    • If a project path from affectedProjects is not in configuredProjects, log and continue.
    • Existing behavior for valid projects is unchanged (mark dirty and clear dirtyFilePath).

• Regression coverage

  • Added TestProjectCollectionBuilder_HandlesStaleConfigRetainer in:
    • internal/project/projectcollectionbuilder_panic_repro_test.go
  • Test injects a stale retained project path for a config entry, triggers a config watch change, and verifies the update path no longer crashes.

for projectPath := range configChangeResult.affectedProjects {
	project, ok := b.configuredProjects.Load(projectPath)
	if !ok {
		if logger != nil {
			logger.Logf("Skipping stale project %s affected by config change (project may have been removed)", projectPath)
		}
		continue
	}
	project.ChangeIf(/* existing dirty-marking logic */)
}