feat(admin): add Template API for K8s Pool-based sandbox warm path

[Generalwin]

What does this PR do?

Implements Template API for sandbox warm pool management via K8s Pool CRD. Resolves #1310.

Changes

• Template API endpoints (template_api.py): POST/GET/DELETE /apis/envs/sandbox/v1/templates
• K8s Provider (provider.py): Pool informer + create_template/get_template_status/delete_template methods
• SandboxManager (sandbox_manager.py): proxy methods for template operations
• Config (config.py): pool_template field for Jinja2-rendered Pool CRD config
• Constants (constants.py): Pool CRD constants, TEMPLATE_ID_PREFIX = "tpl-", managed-by label
• Proto (request.py/response.py): TemplateCreateRequest/TemplateStatusResponse

Design

• templateID = tpl-{sha256(non-null spec fields)[:16]} — same spec always produces same ID
• Pool CRD is the source of truth (no DB), Informer cache for reads
• 409 Conflict on duplicate create = idempotent
• Status mapping: available > 0 → ready, available == 0, total > 0 → building

Testing

• Unit tests (tests/unit/test_template_api.py): 19 cases — ID generation, Spec, Jinja2 rendering, status mapping, constants
• Smoke tests (tests/smoke/test_template.py): lifecycle (create → wait ready → delete) + idempotent create, with --admin-url and --smoke-image CLI options

pytest tests/unit/test_template_api.py     # 19 passed
pytest tests/smoke/ --smoke-image <image>  # 2 passed