Issue #3586271: Clean up CCC inconsistencies and DX issues before rc1 (part 3)

Description

This MR includes the following changes, each in separate commits

• 3c - final added to 9 framework-instantiated leaf classes with no extension use case.
• 3f - 15 classes converted from traditional constructor + property + assignment to PHP 8.1 constructor promotion.
• 2f - Normalized camelCase local variables and method parameters to snake_case in the 6 primary-target files (AiContextSelector, AiContextRequestFactory, AiContextScopeResolver, AiContextRequest, AiContextRequestParamsData, AiContextResult). Promoted constructor properties were left camelCase per Drupal convention. Public method parameters that are called with named arguments elsewhere in the codebase (e.g. AiContextRequestFactory::getResult(maxTokens: ...)) were also left untouched to avoid an API break.
• 3d (Last 2 commits) - Added AiContextRendererInterface and AiContextScopeResolverInterface, following the existing AiContextSelectorInterface pattern (interface + alias registration in ai_context.services.yml). Internal consumers updated to type-hint against the interfaces. AiContextRequestFactory remains a candidate for a follow-up commit if reviewers want it included.

Testing instructions

Acceptance criteria

• 9 leaf classes marked final; no extension-point classes sealed
• Remaining ~15 traditional constructors converted to promotion; parent::__construct() order preserved in forms
• camelCase locals normalized in Service/Model targets (or sub-tasks filed if split)
• At least one new service interface added following AiContextSelectorInterface pattern (or sub-tasks filed for remaining)
• phpcs --standard=Drupal,DrupalPractice src/ tests/ clean
• phpstan analyze clean on touched files
• Existing test suite passes (Unit, Kernel, Functional)

AI Compliance

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Closes #3586271