fix(tracing): truncate oversized span input/output for OpenAI ingest

[OiPunk]

Summary

Fixes #260 by making OpenAI tracing sanitization fail-safe for oversized span_data.input / span_data.output payloads.

What changed

• Extended _sanitize_for_openai_tracing_api to also sanitize input and output fields (not just generation usage keys).
• Added byte-size based truncation helpers:
  • Strings are truncated with a stable suffix (... [truncated]) while staying under the ingest limit.
  • Non-string oversized payloads are replaced with a compact metadata object (truncated, original_type, preview).
• Kept behavior endpoint-aware: sanitization is only applied for the OpenAI tracing ingest endpoint, existing custom endpoint behavior is unchanged.

Tests

• uv run ruff check src/agents/tracing/processors.py tests/test_trace_processor.py
• uv run mypy src/agents/tracing/processors.py tests/test_trace_processor.py
• uv run pytest tests/test_trace_processor.py -q
• make format-check

Added test coverage for:

• large string truncation
• large non-string replacement
• custom endpoint passthrough
• unchanged small input path
• helper edge-cases (within limit, suffix too large)