fix(openai): keep full multi-message context in input.value

[jjcav84]

What does this PR do?

Fixes INPUT_VALUE silently dropping all but the first message for conversational inputs (#151).

_process_input_data wrote INPUT_VALUE three times:

1. JSON dump of the filtered structured messages
2. Joined text of every message's content (eval_input)
3. eval_input[0] — the first message's text only ← the bug

Write 3 is dead logic (its if is implied by write 2's) and destroyed the multi-message context write 2 produced. Removed so the joined string survives; image parts still surface as {{input_images.N}} placeholders with INPUT_IMAGES set alongside.

process([{"role": "user", "content": "a"},
         {"role": "assistant", "content": "b"},
         {"role": "user", "content": "c"}], span)
# before: INPUT_VALUE == "a"
# after:  INPUT_VALUE == "a \n b \n c"

Sibling note for reviewers: traceai_litellm carries the identical pattern — _process_messages returns query = eval_input[0] and _instrument_func_type_completion writes it last, overwriting the joined eval_input. Same first-message-only loss. I kept this PR scoped to the reported path; happy to follow up there if you'd like it fixed identically.

Why?

Closes #151 — trace correctness for multi-message inputs: every message after the first was invisible in input.value.

How was it tested?

• Unit tests added — TestProcessInputData in python/tests/test_framework_openai.py: multi-message preservation, multimodal text+image placeholders, single message, non-list input
• pytest tests/test_framework_openai.py::TestProcessInputData — 4/4 pass
• Pre-existing suite failures unchanged: TestOpenAIFramework fails identically on clean main — its httpx.Client.send mock no longer intercepts under openai 3.x (real API call → 401); unrelated to this diff

Checklist

• Branch is off main
• Conventional Commits (fix(openai): …)
• No TODOs or commented-out code
• No secrets in the diff