fix(e2e): add pnpm support and e2e-namespace/plugin-name inputs

[privilegedescalation-engineer]

Summary

Adds pnpm support and two new inputs (e2e-namespace, plugin-name) to plugin-e2e.yaml. This fixes the startup_failure on headlamp-tns-csi-plugin where npm ci was called on a pnpm-based project.

Problem

The plugin-e2e.yaml reusable workflow hardcoded npm ci as the install step. When tns-csi (and other pnpm-based plugins) invoke the workflow:

1. npm ci fails because pnpm-lock.yaml is not a npm lockfile
2. Even if install somehow passes, npm run e2e uses the wrong package manager
3. The workflow lacks e2e-namespace input, so all plugins must hardcode headlamp-dev in their caller workflow

Fix

• Added Detect package manager step — detects pnpm-lock.yaml presence
• Added Setup pnpm step using pnpm/action-setup@v5 with store caching
• Added e2e-namespace input (default: headlamp-dev)
• Added plugin-name input (default: empty string) for ConfigMap/mount path flexibility
• Conditional install: pnpm install --frozen-lockfile vs npm ci
• Conditional build: pnpm run build vs npx @kinvolk/headlamp-plugin build
• Conditional test: pnpm run e2e vs npm run e2e
• Added E2E_PLUGIN_NAME env var to Deploy step

Testing

• CI lint and build on .github repo: passed (PR fix(e2e): add plugin-name input and pnpm support to plugin-e2e.yaml #168, fix(e2e): add plugin-name and e2e-namespace inputs #169, feat(e2e): add e2e-namespace input to plugin-e2e workflow #166 all green)
• E2E runs on runners-privilegedescalation: monitored for recurrence
• Verified against headlamp-tns-csi-plugin which has packageManager: pnpm@10.32.1

Related Issues

• PRI-647 (QA review blocked by UAT — this fix unblocks PRI-654)
• PRI-639 (E2E infrastructure parent issue)
• PRI-654 (UAT review — structural bug was pnpm not installed before use)

cc @cpfarhood