fix: proxy context-aware methods to target block

[e0ipso]

Summary

• Override context-aware methods on ProxyBlock so context lookups, mappings, and definitions delegate to the configured target block.
• Replace the in-form context mapping element with a warning explaining that context mapping is only available after the block is saved.

Test plan

• Place a context-aware block (e.g. one requiring a node context) via the proxy block in Layout Builder and confirm it renders with the correct context.
• Add a new proxy block targeting a context-aware block and verify the warning is displayed before save.
• Edit the saved proxy block and confirm the context mapping form becomes available.

[gemini-code-assist]

Code Review

This pull request implements context mapping support for the ProxyBlock plugin and updates the TargetBlockFormProcessor to handle context definitions. The code reviewer identified several critical issues in ProxyBlock.php, including a fatal runtime error caused by using the null coalescing assignment operator (??=) on a method return value, a bug where the literal string 'name' is passed instead of the $name variable in getContextDefinition(), and an incorrect fallback array key (context_definition instead of context_definitions) in getContextDefinitions(). Additionally, it was recommended to use $this->t() instead of the global t() function in TargetBlockFormProcessor.php for translation consistency.

[github-actions]

🎉 This PR is included in version 1.6.5 🎉

The release is available on GitHub release

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