* feat(cmdutil): add X-Cli-Build header for CLI build classification
Adds X-Cli-Build (official / extended / unknown) so the gateway can distinguish official CLI from ISV-repackaged builds.
* test(cmdutil): lift coverage on build-kind classification
Extract classifyBuild as a pure helper so every branch (unknown / extended
main-path / extended credential / extended transport / extended fileio /
official) is reachable without mutating process-wide provider registries.
Also cover: isBuiltinProvider non-pointer values, BuildHeaderTransport
nil-Base fallback path, and fix the Name-spoof test so the test double
returns a value that actually mimics an ISV provider.
Coverage on PR-changed functions:
- classifyBuild: 100% (new)
- computeBuildKind: 61.5% -> 93.3%
- BuildHeaderTransport.RoundTrip: 80% -> 100%
