Engine: Delegate context-aware expressions to overridable engine methods

[marcoroth]

This pull request moves expression handling logic from the compiler into overridable engine methods, making Herb::Engine an easier drop-in replacement for Erubi::Engine.

Previously, the compiler bypassed the engine's add_expression method in two cases:

1. Block expressions (<%= link_to do %>...<% end %>), the compiler called add_expression_block which dispatched directly to add_expression_block_result, skipping any subclass override of add_expression.

2. Context-aware expressions (inside <script>, <style>, or attribute values), the compiler's add_context_aware_expression wrote directly to @engine.src, bypassing the engine entirely (see Engine: Quotes inside <script> elements get forcibly entity-escaped in ReActionView #1419)

This meant subclasses that overrode add_expression (the Erubi pattern used by ActionView) would not have their override apply to blocks or context-aware expressions, leading to incorrect compiled output.

So the idea of this pull request is to move the logic from the compiler into overridable engine methods so that subclasses (like ReActionView) get correct behavior by only overriding add_expression. The compiler becomes a thin dispatcher that routes tokens to the engine and no longer manipulates @engine.src directly.

Resolves #1419

Related marcoroth/reactionview#78
Related marcoroth/reactionview#80
Related marcoroth/reactionview#83

[pkg-pr-new]

npx https://pkg.pr.new/@herb-tools/formatter@1421

npx https://pkg.pr.new/@herb-tools/language-server@1421

npx https://pkg.pr.new/@herb-tools/linter@1421

commit: c2d2bbf

[github-actions]

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