feat(openai-messages)!: emit invoke_agent, chat and execute_tool spans

One flat span named openai.response becomes the tree the TypeScript SDK emits:
an invoke_agent root, one `chat {model}` child per model turn, one
`execute_tool {name}` child per tool call.

BREAKING CHANGE: the span this handler emits is renamed from `openai.response`
and `openai.response.stream` to `invoke_agent`. Queries selecting on the old
names will not match. Prompt and completion content is no longer on spans
unless the caller passes capture_content=True.

Cached tokens were absent entirely. OpenAI reports them under
input_tokens_details.cached_tokens, which nothing here read, so every span
understated what a prompt-cached call actually reused. They are now reported in
gen_ai.usage.cache_read.input_tokens and, unlike Anthropic, not added on top of
the input figure: OpenAI already counts them inside it, and adding them would
double-count. Cache creation is always zero, because OpenAI has no such
concept.

This handler is the only one of the six that reports the model which actually
answered rather than the one requested, on both the root and the chat spans.
OpenAI resolves an alias like gpt-4o to a dated snapshot, and this handler has
the resolved value to hand.

Finish reasons are derived, not mapped. The Responses API has no finish_reason
field, so the shared mapping table does not apply and is deliberately not
imported. The value comes from a closed three-way check: a function call in the
output means tool_calls, an incomplete status means length or content_filter
depending on the reported cause, a completed status means stop, and anything
else writes no attribute at all. The function-call check comes first because
status alone reports completed for a turn that stopped to call a tool.

The streaming path gets a finally, so a consumer that breaks out of the
iteration no longer leaves the root span unended and unexported, taking the
whole run out of AI Config Monitoring along with the feature_flag event it
carries.

Tests: 62 to 80.

Author: apucacao