✨ Add ad/creative write operations (create, update, delete ads)

[SamPetherbridge]

Summary

The ads command group added in #32 (PR #34) is read-only (list/get). The asa-api-client library actually supports full ad CRUD — this issue tracks adding the write operations.

Ad creation was deliberately deferred from #32 because, unlike the other v5 resources, it's a real feature with meaningful UX decisions (creative type selection, CPP linking), not just a surfacing pass.

Library surface (already available)

client.campaigns(c).ad_groups(a).ads inherits base CRUD:

• create(AdCreate) -> Ad
• update(ad_id, AdUpdate) -> Ad
• delete(ad_id) -> None

Models:

AdCreate:  name: str
           creative_type: CreativeType        # CREATIVE_SET | CUSTOM_PRODUCT_PAGE | DEFAULT_PRODUCT_PAGE
           status: AdStatus = ENABLED
           product_page_id: str | None        # required for CUSTOM_PRODUCT_PAGE

AdUpdate:  name: str | None
           status: AdStatus | None

Proposed commands

• asa ads create -c <campaign> -a <ad-group> --name <name> --creative-type <type> [--product-page <id>] [--status ENABLED|PAUSED]
• asa ads update <ad-id> -c <campaign> -a <ad-group> [--name <name>] [--status ...]
• asa ads delete <ad-id> -c <campaign> -a <ad-group> [--yes]

Design considerations

1. Creative type ↔ product page coupling. CUSTOM_PRODUCT_PAGE requires --product-page <id>; DEFAULT_PRODUCT_PAGE/CREATIVE_SET must not take one. Validate this combination client-side with a clear error rather than letting the API 400.
2. Discoverability. --product-page ids come from asa product-pages list (already shipped) — cross-reference in help text. Consider validating the CPP belongs to the campaign's app.
3. Confirmation + dry-run. delete should confirm (reuse confirm_action, with --yes to skip); create could support --dry-run to preview the payload, consistent with optimize/translate.
4. CREATIVE_SET scope. Creative sets involve asset/media selection beyond a single id — confirm what the API expects before supporting that type, or restrict the first cut to the product-page creative types.

Acceptance criteria

• create, update, delete implemented following existing typer/utils conventions
• Creative-type/product-page validation with friendly errors
• delete confirmation + --yes; create --dry-run
• mypy --strict, ruff, and tests green; help/registration tests added

Context

Follows #32 (read-only v5 surfacing) and #30 (asa-api-client 0.2.1 adoption).