fix(adapters): support Responses API text content parts

[mikemikimike]

Closes: #1589

Description

Accept OpenAI Responses API EasyInputMessage content arrays containing input_text parts. The request model now validates these parts, and the conversion path preserves their order as BeeAI text content for user, system, developer, and assistant messages. Existing string and None content behavior is unchanged.

Validation

• python -m pytest tests/adapters/openai/test_responses_utils.py -m unit -q -o addopts= — 7 passed
• python -m ruff check --quiet — passed
• python -m ruff format --quiet --check — passed
• python -m pyrefly check beeai_framework/adapters/openai/serve/responses/_types.py beeai_framework/adapters/openai/serve/responses/_utils.py tests/adapters/openai/test_responses_utils.py — 0 errors
• python -m build — passed
• Broader unit run excluding the Transformers-only module: 292 passed, 6 skipped; 4 unrelated Windows platform failures remain because the tests invoke Unix commands/paths.

Checklist

• Python contributor guide reviewed
• Commit is signed off with DCO
• Conventional commit title and message
• Regression test included
• No external API or model call required