refactor(ui/alerts): address code-review findings on alerting-enums

Follow-up to 83837ada addressing the critical-review feedback:

- Duplicate ConditionKind type consolidated: the one in
  api/queries/alertRules.ts (which was nullable — wrong) is gone;
  single source of truth lives in this module.

- Module moved out of api/ into pages/Alerts/ where it belongs.
  api/ is the data layer; labels + hide lists are view-layer concerns.

- Hidden values formalised: Comparator.EQ and JvmAggregation.LATEST
  are intentionally not surfaced in dropdowns (noisy / wrong feature
  boundary, see in-file comments). They remain in the type unions so
  rules that carry those values save/load correctly — we just don't
  advertise them in the UI.

- JvmAggregation declaration order restored to MAX/AVG/MIN (matches
  what users saw before 83837ada). LATEST declared last; hidden.

- Snapshot tests for every visible *_OPTIONS array — reviewer signal
  in future PRs when a backend enum change or hide-list edit
  silently reshapes the dropdown.

- `toOptions` gains a JSDoc noting that label-map declaration order
  is load-bearing (ES2015 Object.keys insertion-order guarantee).

- **Honest about the springdoc schema quirk**: the generated
  polymorphic condition types resolve to `never` at the TypeScript
  level (two conflicting `kind` discriminators — the class-name
  literal and the Jackson enum — intersect to never), which silently
  defeated `Record<T, string>` exhaustiveness. The previous commit's
  "schema-derived enums" claim was accurate only for the flat-field
  enums (ConditionKind, Severity, TargetKind); condition-specific
  enums (RouteMetric, Comparator, JvmAggregation, ExchangeFireMode)
  were silently `never`. Those are now declared as hand-written
  string-literal unions with a top-of-file comment spelling out the
  issue and the regen-and-compare workflow. Real upstream fix is a
  backend-side adjustment to how springdoc emits polymorphic
  `@JsonSubTypes` — out of scope for this phase.

Verified: ui build green, 56/56 vitest pass (49 pre-existing + 7
new enum snapshots).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/src/pages/Alerts/RuleEditor/form-state.ts

@@ -1,10 +1,9 @@
 import type {
   AlertRuleRequest,
   AlertRuleResponse,
-  ConditionKind,
   AlertCondition,
 } from '../../../api/queries/alertRules';
-import type { Severity, TargetKind } from '../../../api/alerting-enums';
+import type { ConditionKind, Severity, TargetKind } from '../enums';
 
 export type WizardStep = 'scope' | 'condition' | 'trigger' | 'notify' | 'review';
 export const WIZARD_STEPS: WizardStep[] = ['scope', 'condition', 'trigger', 'notify', 'review'];