refactor: replace type mixed with more specific types

[paulbalandan]

Description

Fixes #6310.

Reviews every mixed PHPDoc (and its long-form spelling array|bool|float|int|object|string|null) across system/, and for each one decides whether mixed is appropriate, narrows it when the runtime type is provable, or collapses the verbose union to a plain mixed when the value is genuinely any type. Where the chosen type contains array, key/value information is added (array<int|string, mixed>) instead of a bare array. Split into two commits:

1. Narrowing + array refinements

• Narrowed mixed to deducible types.
• Refined bare array to array<key, value> in kept narrower unions.
• Removed a redundant duplicate @param mixed $condition in ConditionalTrait (the @template-typed tag remains).
• Left genuinely any-type sites as mixed, and kept PSR-3 / PHPUnit contract params as mixed to honor those interfaces.

2. Collapsing verbose unions

• Collapsed about 90 long-form array|bool|float|int|object|string|null unions (genuinely any-type) to bare mixed, mostly in Validation/StrictRules, plus Common, Database escape/magic methods, Entity/CastInterface, HTTP request accessors, the array helpers, Model, and View\Parser.
• Kept precise non-mixed unions: the json_decode stdClass shapes and Parser::objectToArray()'s return, which provably excludes object.
• Two call sites that relied on the unions being treated as uncertain under treatPhpDocTypesAsCertain: false were made explicit with !== null.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide