Add embedding generation support

[Jameswlepage]

Fixes #130

Summary

• add embedding DTOs/results/operations and expose helper transformers
• extend PromptBuilder, AiClient, and the CLI with embedding-specific inputs, result helpers, and output formats
• introduce embedding contracts in the provider layer plus an OpenAI embedding model implementation alongside updated mocks/tests/documentation

Testing

• composer phpstan
• composer phpunit
• php cli.php "["Embed this doc"]" --capability=embeddings --providerId=openai --modelId=text-embedding-3-large --outputFormat=embeddings-vectors

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Co-authored-by: Jameswlepage <[email protected]>

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