[runtime] Introduce AI model client function API

[haruki-830]

This PR introduces a generic AI model client API and adds support for using AI functions in YAML pipeline jobs:

transform:
  - source-table: db.users
    projection: ID, AI_COMPLETE('chat_model', CONTENT, 'Complete the text') AS COMPLETED

pipeline:
  model:
    - name: chat_model
      type: openai-compatible
      options:
        model-name: your-model
        endpoint: https://example.com/v1
        api-key: your-api-key

The model client is discovered through the common Factory SPI and validated with FactoryHelper. It is created during pipeline translation and follows the open/close lifecycle of the transform operator.

This PR provides the generic model API and runtime integration. The concrete OpenAI-compatible model client will be introduced in a follow-up PR.

The new API is added alongside the existing Model API. No existing model API is removed. Framework changes are limited to the following:

• Added AiModelClient, ModelContext, AiModelClientFactory, and capability interfaces for text generation and embedding.
• Added factory-based YAML model definitions using name, type, and options.
• Kept compatibility with the existing model-name/class-name configuration and legacy model UDF API.
• Added the generic AI_COMPLETE and AI_EMBED functions.
• Added validation for constant model arguments, undeclared models, invalid model names, and duplicate declarations.
• Added a Dummy model for testing, together with YAML parser, Factory, Calcite/Janino, runtime, MiniCluster, and E2E coverage.

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