Add embedding generation contracts

[chubes4]

Summary

• Add first-class embedding generation contracts and an EmbeddingResult DTO.
• Add OpenAI-compatible embedding request/response handling for providers to reuse.
• Add prompt builder helpers for embedding generation and embedding dimensions.

Fixes #242
Unblocks WordPress/ai-provider-for-openai#32

Testing

• composer test:unit -- --filter 'EmbeddingResultTest|AbstractOpenAiCompatibleEmbeddingGenerationModelTest'
• composer phpstan
• composer phpcs

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (gpt-5.5)
• Used for: Drafted the embedding contract implementation and tests; Chris remains responsible for review and acceptance.

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[chubes4]

Controller triage note: this PR has a PHP 7.4 test failure while #244 for the same issue is fully green. Please either fix the PHP 7.4 failure and clarify how #243 composes with #244, or close/supersede this PR if #244 is the intended canonical embedding API branch.

[chubes4]

Closing this as superseded by #244, which carries the canonical embedding API branch and is already green. This branch still used the older embedding option shape and had a PHP 7.4 enum fixture failure, so keeping #244 as the single embedding contract source avoids splitting the stack across incompatible contracts.