docs-version-sync: nvcaVersion replacement is a silent no-op

Describe the bug

syncClusterManagementSelfManaged in tools/docs-version-sync/inline.go still
syncs a nvcaVersion: key into docs/user/cluster-management/self-managed.md,
but that key was removed from the page in 799a7670c. The replacement matches
nothing, and the returned count is assigned and never checked, so the sync
reports success:

updated, count = replaceYAMLStringValue(updated, "nvcaVersion", nvca.Version)
return updated, updated != content, nil

Every other replacement in the same function either hard-fails on count == 0
(the version table) or discards the count explicitly with _
(replaceHelmVersionArgument). This one is the only silent case. staticcheck
reports SA4006: this value of count is never used.

TestSyncInlineSelfManagedNVCAOperatorVersions passes because its fixture
contains a nvcaVersion: line that no longer exists in the real doc.

Net effect: the NVCA agent version is not synced into any current user doc, and
docs-version-sync gives no warning.

Steps or code to reproduce bug

grep -c nvcaVersion docs/user/cluster-management/self-managed.md   # 0

The only remaining occurrences are in the archived docs/v0.5/ snapshot.

Expected behavior

Needs a docs-ownership decision, so no PR is attached:

• If the NVCA agent version should still be pinned on that page, restore the
  nvcaVersion: key and make the count a hard failure like the version table.
• If the page intentionally dropped it, remove the dead replacement and the now
  unused nvca artifact lookup, and drop the fixture line from the test.

Read syncClusterManagementSelfManaged in tools/docs-version-sync/inline.go, then inspect TestSyncInlineSelfManagedNVCAOperatorVersions and the self-managed documentation fixture. First confirm the current document and fixture contents, then resolve whether the NVCA version belongs on the page; done means the chosen behavior is reflected in the replacement, fixture, and tests, with staticcheck reporting no unused count.