scaffold generated CLI integration point in main.rs

[MintsInc]

Context

openapi-transformer will regenerate pup CLI command modules from the OpenAPI spec into src/generated.rs — #651 is the pilot run for this pipeline. This PR lands the main.rs wiring ahead of that so the integration point is stable: every future openapi-transformer regeneration only adds variants to GeneratedCommand (and sibling modules) inside generated.rs, and main.rs never needs to change again.

Changes

Added mod generated alongside the other top-level module declarations, and flattened GeneratedCommand into the Commands enum with #[command(flatten)] rather than nesting it under a pup generated subcommand — generated subcommands will appear as top-level peers (e.g. pup downtime get ...), matching how every other domain is exposed today. Routed the new arm through main_inner() following the existing per-domain pattern (cfg.validate_auth() then dispatch).

src/generated.rs here is a placeholder with an empty GeneratedCommand enum, not #651's actual generated output. #651 introduces a generated Downtime variant that collides on the subcommand name with pup's existing hand-written top-level downtime command (list/get/create/cancel): clap panics at runtime with "command name downtime is duplicated" on any invocation that builds the full Command tree. That collision needs a product decision — retire/merge the hand-written downtime command, or pick a different pilot operation upstream in datadog-api-spec — before any generated variants can land. This PR only establishes the scaffold; once that decision is made, #651 becomes a pure additive diff to generated.rs and its sibling modules, with no main.rs changes required.

Tests

• cargo build, cargo clippy --all-targets -- -D warnings, and cargo fmt --check all pass.
• cargo test passes except 4 pre-existing failures unrelated to this change (DNS lookup failures against unused.local in commands::cases, commands::dbm, commands::monitors, commands::traces tests) — confirmed identical on a clean main checkout.
• Manually confirmed pup --no-agent --help renders without panicking (no subcommand collision, since the placeholder enum has zero variants).