rc.7 will be the first RC in the 1.39.0 train that actually rolls in
the post-rc.5 fixes from main (rc.6 was content-identical to rc.5 — see
#2856). Notes enumerate each fix with PR/issue link, recap rc.6 / rc.5 /
rc.4, and follow the established docs/RELEASE-v1.39.0-rc.X.md format.
No SDK-version pinning advice (consistent with the rc.6 doc cleanup).
Markdownlint-clean fenced code blocks.
Closes#2859
