Fix PHPStan level-6 warnings (install hooks, Views filter plugins, test assertions)

Problem/Motivation

#3605324 fixed the PHPStan config so the CI job now actually runs. It found 17
pre-existing level-6 violations: nullable return types on install hooks that never
return null, type mismatches in the Views filter plugins (getValueOptions,
getAvailableYears, getMonthOptions), and a handful of pedantic test assertions
(assertTrue(true), assertIsInt on a known int, etc.). Full list in the
phpstan-baseline.neon artifact on MR !16.

Proposed resolution

Fix the violations in source and test files:

• ai_metering.install: narrow ?string to string on hook_update_9008/9010/9011.
• UsageModelFilter, UsageProviderFilter: match getValueOptions() return type to parent (array|null), remove redundant isset, drop unreachable code.
• UsagePeriodFilter: fix array key types on getAvailableYears() / getMonthOptions(); annotate $this->query as Sql so PHPStan resolves addWhere().
• Tests: type mock variables as MockObject&CacheBackendInterface; remove always-true assertions.

Remaining tasks

• Fix all 17 violations, confirm PHPStan CI goes green.

User interface changes

None.

API changes

None.

Data model changes

None.

AI assistance

N/A