Core: Add framework-neutral model client adapters

[Minseok Song (skytin1004)]

Purpose

Introduce a framework-neutral model client boundary so Co-op Translator can evaluate Microsoft Agent Framework without exposing framework-specific types throughout the translation pipeline. Semantic Kernel remains the default backend.

Description

• Add TranslationModelClient and ModelResponse as the minimal internal completion contract.
• Add Semantic Kernel and Microsoft Agent Framework adapters.
• Select the experimental adapter with CO_OP_TRANSLATOR_MODEL_CLIENT=agent-framework; invalid values fail clearly.
• Route Azure OpenAI and OpenAI Markdown translation and LLM evaluation through the shared contract while preserving credential-set fallback.
• Allow injected Agent Framework clients, including Anthropic and Ollama connectors, to bypass OpenAI-specific credential handling.
• Add current Agent Framework core/OpenAI runtime dependencies and development-only Anthropic/Ollama connectors.
• Document backend selection and explicitly keep image structured-output translation on its existing OpenAI SDK path.

Related Issue

N/A

Does this introduce a breaking change?

Will this change require users to update commands, configuration, environment variables, generated translation output, or public Python/MCP APIs?
If you're not sure, test the affected CLI, API, documentation, or GitHub Actions workflow before marking this as "No."

• Yes
• No

Type of change

• Bugfix
• Feature
• Code style update (e.g., formatting, local variables)
• Refactoring (no functional or API changes)
• Documentation content changes
• Other... Please describe:

Checklist

Before submitting your pull request, please confirm the following:

• I have thoroughly tested my changes: I confirm that I have run the code and manually tested all affected areas.
• All existing tests pass: I have run all tests and confirmed that nothing is broken.
• I have added new tests (if applicable): I have written tests that cover the new functionality introduced by my code changes.
• I have followed the Co-op Translator coding conventions: My code adheres to the style guide and coding conventions outlined in the repository.
• I have documented my changes (if applicable): I have updated the documentation to reflect the changes where necessary.

Additional context

Validation completed:

• Python 3.12 full suite: 381 passed
• Black, Ruff, and mypy checks pass
• Poetry lock consistency and pip check pass
• Live configured-provider smoke tests pass for both Semantic Kernel and Agent Framework direct completion
• Live Agent Framework Markdown translation, Python API, CLI, and MCP translation pass
• Agent Framework's Ollama connector completed a direct request against a separate CPU-only local server
• The local phi4-mini:3.8b model did not preserve Co-op Translator's chunk envelope during a full Markdown translation, so Ollama model quality/compatibility is not claimed by this PR
• Anthropic connector compatibility is covered with the real connector class and a mocked network response; no live Anthropic test was run because no credential was available

This PR intentionally retains Semantic Kernel and keeps it as the default. It establishes the comparison boundary; it does not yet remove Semantic Kernel or promise production support for every Agent Framework provider.

[Minseok Song (skytin1004)]

CI status note: Python 3.11-3.14, the Python 3.14 requirements check, CodeQL, and documentation build all pass. The Co-op review check reports 110 structure errors because the two changed documentation files (AGENTS.md and docs/configuration.md) do not have corresponding files in each of the 55 translation directories. This is documentation-translation coverage, not a model-client test or runtime failure; I am leaving the PR in Draft while that repository-wide documentation policy is considered.