chore(quality): fix release-tip base-reds — providerTierField case collision + stryker 7806 registration

(1) #7838 added providerTierField.ts next to ProviderTierField.tsx in the same
directory — a case-only collision that breaks webpack on case-insensitive
filesystems; the #6584 guard fails Unit shard 4/4 on every fresh PR run.
Rename the helper to providerTierFieldApi.ts (import + test path adjusted).
(2) check:mutation-test-coverage --strict fails on the tip because merged
#7806's combo-skip-conn-disable-plugin-block test was never registered in
stryker tap.testFiles. Register it.

src/app/(dashboard)/dashboard/providers/[id]/components/modals/ProviderTierField.tsx

@@ -4,7 +4,7 @@ import { useEffect, useState } from "react";
 import { useTranslations } from "next-intl";
 import { Select } from "@/shared/components";
 import type { ProviderTier } from "@omniroute/open-sse/services/tierTypes";
-import { fetchProviderTierOverride, saveProviderTierOverride } from "./providerTierField";
+import { fetchProviderTierOverride, saveProviderTierOverride } from "./providerTierFieldApi";
 
 export interface ProviderTierFieldProps {
   /** Provider connection id string — the same key `classifyTier()` matches on. */