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 private-method parameters to snake_case in the 6 files named in the issue (AiContextSelector, AiContextRequestFactory, AiContextScopeResolver, AiContextRequest, AiContextRequestParamsData, AiContextResult), with two corrections from an earlier version of this description: AiContextRequest is a pure value object with only promoted constructor properties, so it had no applicable locals to normalize; and AiContextResult's one parameter rename (getSelectedItems($fullData)) was reverted back to camelCase after review, since it has no interface and the name is part of its public contract. No public method signature changed as a net result of this item — every parameter reachable via named arguments (AiContextRequestFactory::fromAgent()/isLoopAware()/findAgentConfig()/getResult()/getRenderedContext(), AiContextScopeResolver::prefilterItemIdsByScope()/filterByScope()/scoreAndSort()/calculateScopeScore()) was kept or restored to its original camelCase name. Only local variables and private-method parameters (never externally callable) were renamed.
• 3d (Last 2 commits) - Added AiContextRendererInterface, AiContextScopeResolverInterface, following the existing AiContextSelectorInterface pattern (interface + alias registration in ai_context.services.yml). Internal consumers updated to type-hint against the interfaces. AiContextRequestFactory included later in separated commit 205e1a37.

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