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.

tests/unit/provider-tier-field-helpers.test.ts

@@ -7,7 +7,7 @@ import test from "node:test";
 import assert from "node:assert/strict";
 
 const { normalizeTierValue, fetchProviderTierOverride, saveProviderTierOverride } = await import(
-  "../../src/app/(dashboard)/dashboard/providers/[id]/components/modals/providerTierField.ts"
+  "../../src/app/(dashboard)/dashboard/providers/[id]/components/modals/providerTierFieldApi.ts"
 );
 
 type FetchCall = { url: string; init?: RequestInit };