The context and one-shot suites duplicated cancellation file markers
and filtering of registrar-owned runtime contexts.
Move those mechanics into `tests._helpers` while retaining each
endpoint's distinct startup handshake. Also update the startup-cancel
`Channel.send()` mock to accept and forward the new `send_deadline` arg.
Caught-during: review remediation
Found-via: `/run-tests` test_cancel_during_context_startup[trio]
Review: PR #481 (goodboy)
https://github.com/goodboy/tractor/pull/481#pullrequestreview-5012942328
(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))
Gud Boi