Replace the two stubbed tests with a single test that drives the real
CodexTransport.build_kwargs (the fixture agent already carries a tool), so
the test binds the summary body actually sent, not a hand-written dict. It
covers both the first attempt and the empty-summary retry; the previous
pair asserted the same three keys twice and reused one dict across
attempts, so the second-attempt check passed trivially after the first pop.
Add the WHY comment on the pops: the transport emits tools, tool_choice and
parallel_tool_calls as one block, and strict Responses backends 400 on the
controls without tools.
476a45f4f3