refactor(core): move provider env key exceptions into plugins

[rekram1-node]

Closes #44065

models-dev.ts                      before: environmentNames() with per-provider special cases
                                   after:  every models.dev env var is registered as a key (no provider IDs)

provider plugins narrow providers whose models.dev env also lists setup values (method.update env)
  databricks             (new plugin) → DATABRICKS_TOKEN          DATABRICKS_HOST only fills the base URL
  neon                   (new plugin) → NEON_AI_GATEWAY_TOKEN     NEON_AI_GATEWAY_BASE_URL only fills the base URL
  google-vertex          (moved)      → GOOGLE_VERTEX_API_KEY
  azure                  (moved)      → AZURE_API_KEY, AZURE_COGNITIVE_SERVICES_API_KEY
  cloudflare-workers-ai  (moved)      → CLOUDFLARE_API_KEY, CLOUDFLARE_WORKERS_AI_TOKEN, CLOUDFLARE_API_TOKEN
  amazon-bedrock, snowflake-cortex    → unchanged

accepted env names vs v2
  databricks  DATABRICKS_HOST, DATABRICKS_TOKEN               → DATABRICKS_TOKEN        (HOST was active first)
  neon        NEON_AI_GATEWAY_BASE_URL, NEON_AI_GATEWAY_TOKEN → NEON_AI_GATEWAY_TOKEN   (BASE_URL was active first)
  every other provider                                        → unchanged