feat(BA-5502): Support runtime_variants list in deployment config

[jopemachine]

resolves #10660 (BA-5502)

Summary

• Change DeploymentConfig.runtime_variant (single value) to runtime_variants (list)
• User's explicit choice always takes priority; validated against the list if present
• When user does not specify (Sentinel.TOKEN default):
  • Single variant in list → auto-selected
  • Multiple variants → RuntimeVariantNotSpecified error
  • No list → falls back to CUSTOM
• Both runtime_variant = "vllm" (legacy) and runtime_variants = ["vllm", "sglang"] accepted in deployment config TOML
• Resolution logic consolidated into DeploymentConfig.resolve_runtime_variant()
• RuntimeVariantNotAllowed for invalid choice, RuntimeVariantNotSpecified for missing choice

Example deployment-config.toml

runtime_variants = ["vllm", "sglang"]

[environment]
image = "default-image:latest"
architecture = "x86_64"

[vllm]
resource_slots = { cpu = 8 }

[sglang]
resource_slots = { cpu = 12 }

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation