fix: stop overwriting multi-message INPUT_VALUE in OpenAI instrumentor

[nigamgauri]

What does this PR do?

Fixes a bug in the OpenAI instrumentor where multi-turn conversations lost every message after the first one in the trace.

Why?

_process_input_data in _span_io_handler.py set INPUT_VALUE on the span three times. The first write was correct, a JSON array of every message in the conversation. The next two writes overwrote it: first with a joined string of all message text, then with just eval_input[0], the text of the first message only. Any conversation with more than one message lost everything past turn one in the trace. There's no EVAL_INPUT attribute defined in fi_types.py, so the eval_input accumulation had no other consumer, it was just clobbering good data. Fixes #151.

How was it tested?

• Unit tests added (pytest)
• Integration tests pass (N/A, no gateway behavior changed)
• ruff check passes on the files I changed
• mypy was not run cleanly on this branch (pre-existing errors in unrelated files, unrelated to this fix)
• Public types, env vars, or SDK behavior documented in README

Added tests/test_span_io_handler.py with two cases: a plain multi-turn conversation, and a multimodal message list with text and an image. Both fail on main (the multi-turn case raises a JSONDecodeError since INPUT_VALUE ends up as raw text instead of JSON) and pass with the fix.

Checklist

• Branch is off main
• Commit messages follow Conventional Commits
• No TODOs or commented-out code left in
• No real API keys or secrets in the diff
• N/A, no prose changes

Notes for reviewers

I didn't touch the multimodal branch's image handling, only removed the three-way overwrite on INPUT_VALUE. Happy to add a changelog entry if the project wants one for this package.