gate hand-written downtime list for generated takeover

[MintsInc]

Context

pup downtime is moving from hand-written to generated-from-spec commands one operation at a time. get, create and cancel are already generated; list is the last hand-written verb. Once the generated list lands, the hand-written one must compile out, otherwise clap panics with Command downtime: command name 'list' is duplicated.

Changes

• Gates the hand-written list variant, dispatch arm, handler, its ListDowntimesOptionalParams import and its test behind cfg(not(generated_op = "downtime.list")), the same pattern used for get/create/cancel. This is a no-op until the generated module includes list.
• With all four ops generated, nothing in src/commands/downtime.rs is compiled and its shared imports would fail clippy -D warnings. The module declaration is gated on all four ops rather than repeating the gate on each import. This gate only covers the all-generated case. Taking list back out of the generated surface returns to today's state and compiles, but reverting an already-generated op (create, cancel) on its own is not supported and would trip unused-import warnings. That is acceptable because the hand-written module is deleted in a follow-up once the generated list lands.
• cli_surface_matches_reviewed_projection_for_sample_command now switches the list expectation on cfg!(generated_op = "downtime.list"). The post-migration shape (--current-only, --include, --page-limit, --page-offset) is taken from openapi-transformer pup describe-surface.
• The help-order sort line for downtime is already in place, so no change is needed there.

Tests

• cargo clippy --all-targets -- -D warnings and cargo fmt --check pass.
• Simulated the post-migration state locally: added downtime.list to src/generated/surface.txt and replaced src/generated/downtime.rs with the module rendered by openapi-transformer generate client --generator pup. Without this change, tests that build the CLI panic on the duplicate list. With it, clippy passes, the downtime and surface-projection tests pass, and pup downtime --help lists cancel, create, get, list in order.
• Non-agent output of downtime list with no flags is unchanged: same single ListDowntimes call with default params, same formatter. Agent-mode output gains metadata.count/command.

[datadog-datadog-prod-us1]

In partial-generation configurations (e.g., only create or cancel remaining hand-written), shared and test-support imports in the downtime module become unused, causing cargo clippy --all-targets -- -D warnings to fail and breaking the advertised per-operation fallback. These imports must be gated on the relevant hand-written operations remaining active.

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