feat(cli): auto-create providers for explicit --provider names

[drew]

Summary

• When --provider <name> is passed and no provider with that name exists, the CLI now checks if the name is a recognized provider type (e.g. claude, nvidia, github) and auto-creates it from local credentials
• If the name is not a recognized type, the CLI returns a clear error instead of deferring validation to the server (which returned a cryptic FailedPrecondition gRPC error)
• Extracted shared auto_create_provider() helper used by both explicit --provider names and inferred provider types, eliminating duplication
• Fixed a deduplication bug where the same provider could be created twice when it appeared in both explicit and inferred lists

Motivation

Running nemoclaw sandbox create --provider nvidia -- claude on a fresh cluster failed with:

Error: provider 'nvidia' not found

The --provider flag passed names directly to the server without any client-side validation or auto-creation. This PR makes explicit --provider names follow the same auto-creation flow as inferred providers.

Test Plan

• 7 new integration tests (ensure_providers_integration.rs) against a mock gRPC server covering:
  • Explicit provider name passes through when it exists
  • Explicit provider name auto-creates when it matches a valid type
  • Unrecognized provider name returns a clear error
  • Inferred type auto-creates (existing behavior preserved)
  • --no-auto-providers skips missing providers
  • Explicit + inferred providers combined correctly
  • Deduplication when same provider appears in both explicit and inferred lists
• 1 new e2e test (provider_auto_create.rs) that verifies the full flow end-to-end:
  • Sets ANTHROPIC_API_KEY in the environment
  • Runs nemoclaw sandbox create --provider claude --auto-providers -- printenv ANTHROPIC_API_KEY
  • Asserts the provider was auto-created and the credential is available inside the sandbox
• All existing tests pass (cargo test -p navigator-cli, mise run pre-commit)