refactor(detection-emulation): move resolveCurrentUsername to shared lib

Relocates resolve_current_user.ts from the skill-specific directory
to server/lib/detection_emulation/ so it's importable from both the
Agent Builder tool handlers and the REST routes without a cross-concern
import path.

Previously flagged in the code itself as "should be upstreamed" — this
is the short-term path (shared within the plugin) while awaiting an
export from @kbn/agent-builder-server.

Author: patrykkopycinski

x-pack/solutions/security/plugins/security_solution/server/agent_builder/skills/detection_emulation/validate_rule_tool.ts

@@ -46,7 +46,7 @@ import {
   resolveRateLimiterConfig,
 } from '../../../lib/detection_emulation/runtime_config_resolver';
 import { buildAgentBuilderActor } from '../../../lib/detection_emulation/execution/audit_context';
-import { resolveCurrentUsername } from './resolve_current_user';
+import { resolveCurrentUsername } from '../../../lib/detection_emulation/resolve_current_user';
 import { validateRuleSchema } from './validate_rule_input';
 import { toolError, type EmulationErrorContext } from './emulation_tool_errors';
 

x-pack/solutions/security/plugins/security_solution/server/agent_builder/skills/detection_emulation/with_command_gates.ts

@@ -24,7 +24,7 @@ import { buildIdempotencyKey } from '../../../lib/detection_emulation/execution/
 import type { ActorContext } from '../../../lib/detection_emulation/execution/audit_context';
 import { createSavedObjectRuleBindingLookup } from '../../../lib/detection_emulation/rule_binding_lookup';
 import { emulationRuleBindingTypeName } from '../../../lib/detection_emulation/rule_binding';
-import { resolveCurrentUsername } from './resolve_current_user';
+import { resolveCurrentUsername } from '../../../lib/detection_emulation/resolve_current_user';
 import { toolError, type EmulationErrorContext } from './emulation_tool_errors';
 import {
   checkRealExecutionFeatureFlags,