fix(logging): preserve agent workflow output and finish reasons

[pradystar]

Fix missing output and finish reasons on parent agent/workflow spans.

• Let successful decorated agents and workflows returning None use the logger’s existing child-output fallback.
• Preserve explicit empty outputs and prevent failed operations from inheriting a successful child answer.
• Capture provider finish reasons in streaming and non-streaming OpenAI Chat Completions.
• Propagate the terminal LLM finish reason to enclosing agent/workflow spans when their outputs match.
• Add a banking agent example, regression tests, and documentation.

Validation

• Focused mocked suite: 113 tests passed across decorator ownership, logger OTLP egress, OpenAI extraction/instrumentation, and span conversion.
• Ruff lint and format checks passed for the changed SDK and test files.
• The second banking query completed successfully using the modified SDK without raw OTLP capture.
• Full suite and type checks were not run.
• Parent span output and finish reason still need confirmation in a live trace.

Before - https://app.us1.observability.splunkcloud.com/#/agent-obs/project/f077de91-25d9-4ae3-9f00-3c38fc32ed69/agent-streams/c6e85115-69de-4192-84ef-6256ee3c8bb4/sessions/b08f50d4-eb10-4247-88a0-213ee02af2a8?timeRange=%7B%22type%22:%22lastYear%22%7D&table-filters-store-v1=%7B%22state%22:%7B%22filters%22:%5B%5D,%22filterConnector%22:%22and%22,%22textSearch%22:%22%22,%22lastContextKey%22:%22f077de91-25d9-4ae3-9f00-3c38fc32ed69-c6e85115-69de-4192-84ef-6256ee3c8bb4-%22%7D,%22version%22:0%7D&traceId=1ddada64-c939-472c-afb7-8175826e1c4b&spanId=7dece5b4-ab5e-4b2c-a938-91d338ad577d
After - https://app.us1.observability.splunkcloud.com/#/agent-obs/project/f077de91-25d9-4ae3-9f00-3c38fc32ed69/agent-streams/c6e85115-69de-4192-84ef-6256ee3c8bb4/sessions/0c94352b-1fc8-4173-b54d-9274032e2b5c?timeRange=%7B%22type%22:%22lastYear%22%7D&table-filters-store-v1=%7B%22state%22:%7B%22filters%22:%5B%5D,%22filterConnector%22:%22and%22,%22textSearch%22:%22%22,%22lastContextKey%22:%22f077de91-25d9-4ae3-9f00-3c38fc32ed69-c6e85115-69de-4192-84ef-6256ee3c8bb4-%22%7D,%22version%22:0%7D&spanId=fe8407d4-bc93-4b27-bce8-5bb704fd8bb4&traceId=9c36724b-3964-41f5-8c9b-632a8208d472

[etserend]

Looks good to me.