Reduce PHPStan baseline and fix type debt

What

Reduce and eventually remove the 521-finding PHPStan baseline introduced by #1805 while preserving add-on compatibility rules for public APIs.

Why

The baseline restores a regression gate but represents substantial type debt. Fixing the findings will improve defect detection and make future PHPStan upgrades safer.

Tier

tier:standard — findings span many production components and require semantic type decisions, but the gate and rollback are established.

Files

• phpstan-baseline.neon
• phpstan.neon.dist
• Production files named by baseline entries under inc/, views/, and ultimate-multisite.php

Reference pattern

Run pnpm run stan, fix underlying code or accurate PHPDoc, and remove only the matching count-specific baseline entry. Do not add public return declarations to extensible base classes, interfaces, gateways, integrations, UI base elements, models, or checkout APIs.

Implementation steps

1. Group findings by identifier and prioritize real undefined-variable, argument, and return-contract defects.
2. Apply small coherent fixes with focused regression tests where behavior is uncertain.
3. Regenerate or hand-reduce the baseline only after each fixed group.
4. Keep broad ignore rules and analysis level unchanged unless separately justified.

Acceptance criteria

• Baseline finding count decreases materially.
• No new broad ignore rule is introduced.
• pnpm run stan passes.
• Relevant focused PHPUnit tests pass.
• Public extension compatibility constraints remain intact.

Verification

• pnpm run stan
• pnpm run quality
• Focused PHPUnit classes for changed production code

Dependency

Blocked by #1805; begin after PR #1806 merges.

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After #1805 and PR #1806 are complete, run pnpm run stan and group findings from phpstan-baseline.neon by identifier, tracing entries into inc/, views/, and ultimate-multisite.php. Fix coherent type issues with focused PHPUnit coverage where needed, then remove only matching baseline entries; pnpm run stan, pnpm run quality, and relevant PHPUnit classes must pass without weakening compatibility rules.