Support OpenAI Responses API content parts

[RitwijParmar]

Fixes #1901.

This fills in the Responses API content-part gap in the OpenAI instrumentation:

• input_text now maps to a text part/event instead of falling through as unknown
• input_image handles the Responses API's flat image_url string as well as file_id and inline file data
• input_file maps URL and file-id references to document URI parts, and inline file_data to a document blob part
• the legacy events path now records useful content for these same parts instead of gen_ai.unknown

I kept unknown/future part types on the existing generic fallback, so this should be narrowly scoped to the documented Responses API part types.

Tests run:

• uv run pytest tests/otel_integrations/test_openai.py -q -k "input_to_events or convert_responses_inputs"
• uv run ruff check logfire/_internal/integrations/llm_providers/openai.py tests/otel_integrations/test_openai.py
• uv run ruff format --check --diff logfire/_internal/integrations/llm_providers/openai.py tests/otel_integrations/test_openai.py
• uv run pyright logfire/_internal/integrations/llm_providers/openai.py tests/otel_integrations/test_openai.py

AI disclosure: I used AI assistance while editing and reviewing this patch, then ran the tests and checks above locally.

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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