fix: retain standalone integration traces

[weivwang]

What changed

• retain start_span() calls made outside an explicit trace as queryable one-span traces
• propagate a temporary trace context so nested spans attach to the standalone root
• emit the same trace_start / trace_end lifecycle events used by explicit traces
• add regression coverage for queryability, nesting, error status, and context cleanup

Why

The zero-config OpenAI and Anthropic integrations call start_span() directly. When an application used the documented instrument_all() flow without wrapping every SDK call in start_trace(), the span completed but was never added to _traces. Dashboards and exporters therefore saw no trace even though instrumentation ran successfully.

Impact

Auto-instrumented provider calls now remain visible without requiring an undocumented explicit trace wrapper. Existing spans inside an explicit trace keep their current parent-child behavior.

Validation

• uv run --with pytest --with pytest-asyncio pytest -q — 41 passed
• uv build — wheel and source distribution built successfully
• git diff --check

The repository's existing ruff check agent_trace/ baseline is currently not clean (pre-existing import-order, unused-import, line-length, and Python-3.9-incompatible UP045 findings); this PR does not broaden scope into a repository-wide formatting migration.