Add OTEL tracing to the sampling pipeline:
- `sampling/createMessage` span wrapping sample_impl() with attributes for
temperature, max_tokens, tool_count, result_type, and iteration count
- `sampling/createMessage step` child spans per loop iteration with
iteration index and stop reason
- `sampling.execute_tool <name>` spans for tool calls within sampling,
including error status on failures
- Validation failure and text response retry events on the parent span
- Opt-in content capture via OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT
Also adds is_recording() guards to server_span, delegate_span, and
client_span to avoid unnecessary attribute serialization when sampling is
disabled, and sets error.type using __qualname__ for better nested class
names.
Closes#3891
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
