fix(tracing): reject zero batch sizes

[hsusul]

Summary

• reject max_batch_size=0 during BatchTraceProcessor construction
• prevent force_flush from silently leaving queued spans unexported

Test plan

• UV_CACHE_DIR=/tmp/uv-pr6-b uv run pytest tests/test_trace_processor.py -q (58 passed)
• changed-file Pyright and mypy
• full suite: 9031 passed, 39 skipped; 38 environment-restricted failures (forkserver/native macOS sandbox)

Issue number

N/A

Checks

• Focused regression tests added
• git diff --check clean

[seratch]

We'd like to hold off adding this type of obvious value error validation for now.