Blocks context reaction incompatible with decorated block_manager services (e.g. Canvas)

Problem/Motivation

The Blocks context reaction injects the block_manager service with a concrete BlockManager type hint in its constructor. When another module decorates the block_manager service — as the Canvas module does with its BlockManagerDecorator — Symfony's dependency injection container throws a TypeError because the decorator is not an instance of the concrete BlockManager class, even though it correctly implements BlockManagerInterface. This causes a fatal error on any page load when both Context and Canvas are enabled.

Steps to reproduce

1. Install the Context module (5.0.0-rc2)
2. Install the Canvas module (1.7.1)
3. Enable both modules
4. Visit any page on the site

Proposed resolution

Change the type hint in Blocks::__construct() from the concrete BlockManager class to BlockManagerInterface, and update the corresponding @var docblock and use statement accordingly. Any class that correctly implements BlockManagerInterface — including decorated services — will then be accepted.

Remaining tasks

Review and commit the patch

User interface changes

None

API changes

None. The constructor parameter changes from a concrete class to the interface it implements. Any code already passing a valid BlockManager instance continues to work unchanged.

Data model changes

None