[Platform][OpenRouter] Add Embeddings incl. example, fill Model catalog

[lochmueller]

This commit adds openrouter.ai features:

• Add the current set of models (without capabilities, because they are not checked)
• Add an Embeddings model + client and result converter to support embeddings
• Add an example for embeddings models
• Add error handling in the default ResultConverter of the completions endpoint to handle the "rate limit" of the free tier in a better way.

[chr-hertel]

Happy to see openrouter get some attention - thanks for that 🙏

Left some minor comments, but close for merging 👍

[lochmueller]

@OskarStark thanks for the reviews. What is exactly the problem of the Fabbot pipeline? I have to change the PR header or description.... but in which way? https://symfony.com/doc/current/contributing/code/pull_requests.html#make-a-pull-request Do you have a hint for me? I don't get it...