github: Look up whether a comment landed before resending a mutation

[jerry-skydio]

Github applies mutation fields in order and their effects survive its gateway
giving up, and a transient http failure carries no field results, so replaying
the request posts the comment a second time. A real 502 while uploading a 60 pr
stack duplicated the review graph and patchsets comments on 13 prs, and three
attempts left some prs with three copies each.

Fields now carry a replay_safe flag, and addComment is the only unsafe one:
github allows unlimited identical comments, and echoes clientMutationId rather
than treating it as an idempotency key. Every other mutation is addressed by an
id or is a create that github rejects as UNPROCESSABLE the second time, which
already reads as done.

A request holding an unsafe field is sent once. If that fails transiently, read
back the newest comments of each subject pr and resend only the fields whose
body isn't there yet, within the usual retry budget.

Limitations:

• The check is a read, so a comment that did land can still be duplicated if
  github serves a stale replica, or if 20 newer comments push it out of the
  window being read.
• A comment the check can't prove missing (the pr node came back null, or the
  lookup itself failed) counts as added, leaving it for the next upload to add.
• Exhausting the retry budget raises, so the rest of that mutation and the rest
  of the upload don't run.

[jerry-skydio]

Reviews in this chain:
└#302 github: Look up whether a comment landed before resending a mutation

[jerry-skydio]

#	head	base	diff	date	summary
0	4cbf01c8	cda06a4a	diff	Sep 23 15:50 PM	4 files changed, 238 insertions(+), 10 deletions(-)
1	ff727d74	c7a2fae5	diff	Sep 25 16:55 PM	1 file changed, 4 deletions(-)
2	4ba02662	c7a2fae5	diff	Sep 25 16:58 PM	2 files changed, 14 insertions(+), 22 deletions(-)