Add PowerShell package manager mapping

[MariusStorhaug]

PowerShell smoke tests can now resolve the conventional PowerShell updater image without requiring an explicit --updater-image override.

Changes

• Map package-manager: powershell to ghcr.io/dependabot/dependabot-updater-powershell.
• Cover the generated updater image name in Test_setImageNames.

record_ecosystem_meta remains intentionally excluded from smoke-test recordings and expectations, consistent with #587. This focused registration change does not alter that global contract.

Related work

• Depends on New ecosystem: PowerShell dependabot-core#15666 for the official PowerShell updater image.
• Unblocks ⚙️ [Maintenance]: PowerShell changes gain production-like validation smoke-tests#569 once this change is released.
• A CLI release is required because the official smoke workflows install github.com/dependabot/cli/cmd/dependabot@latest.

Testing

• go test ./internal/infra -run '^Test_setImageNames$/powershell$' -count=1
• go test -shuffle=on -count=2 -cover -timeout=5m ./cmd/dependabot/internal/cmd ./internal/...
• go build -v ./...
• go mod tidy -diff
• go vet ./...
• gofmt -l .
• Upstream build and lint checks pass, including the Linux race-enabled suite.
• Focused code review found no issues.

CI note

The smoke matrix passed 100 jobs. The unrelated python-pip-compile job has expectation drift with only 97/130 cached calls; this mapping does not touch Python or smoke expectations.