fix(ci): match SelfHosted-macOS flag case in codecov carryforward

[mc856]

Contribution path

• Small, safe change that does not need a tracking issue

Problem

External fork PRs get a spurious codecov/project failure whenever the base commit on main has self-hosted macOS coverage. .codecov.yml lists the carryforward flag as SelfHosted-macos, but ci.yml uploads SelfHosted-${{ matrix.os }}, i.e. SelfHosted-macOS. Codecov flag names are case-sensitive, so the macOS report never carries forward on fork PRs, while SelfHosted-Linux and SelfHosted-Large — exact-case names in the same block — carry forward fine. Dates back to the flag split in #1901.

Observed on #3210 (docs-only, so the project total couldn't legitimately move): the head report carries forward Linux ×4 and Large ×2 but no macOS; base 68bb0bd has two SelfHosted-macOS uploads at 74.90%, head lands at 74.62% — the missing 291 of 106,086 tracked lines are exactly the reported −0.28%. Same shape on #3060 (−0.30%, merged over the red X) and #3179 (−1.56%). #3201 and #3181 passed only because their base commits happened to be missing the macOS upload too, so for external contributors the check is effectively a coin flip on the base commit's runner health.

Solution

One-character case fix so the config matches the flag CI actually uploads.

How to Test

Codecov reads the YAML from the head commit, so this PR's own report should list SelfHosted-macOS as carried forward (visible under the uploads on the Codecov page for the head commit), and fork PRs opened after this merges should stop seeing the spurious ~0.3% project drop.

AI assistance

Claude Code with Fable 5: ran the codecov API comparison and drafted this description. I verified the flag names in .codecov.yml/ci.yml and the upload/coverage numbers myself.

Checklist

• I have read and approved the CLA.