refactor: type the generated inventory as a group mapping

[blaipr]

generate_inventory built one dict holding two unrelated shapes: _meta, whose value is host variables, and one entry per group name, whose value is hosts, children and vars. Indexing that dict means the checker sees the union of both, so inv_list[group]['hosts'].append(...) reads as .append on a dict and ty reported three unresolved attributes for lines that are correct at runtime.

The two shapes are now two variables:

class InventoryGroup(TypedDict):
    hosts: list[str]
    children: list[str]
    vars: dict[str, str]

groups: dict[str, InventoryGroup] = {}
hostvars: dict[str, dict[str, str | int]] = {}

They are joined into the Ansible dynamic inventory format on the way out, _meta first and the groups in the order they were met, which is the order the single dict produced before.

Verification

No behaviour change, checked rather than reasoned about. json_inventory and ini_inventory return byte-identical output at 0, 1, 2, 9, 10, 11, 100 and 1000 hosts, compared against main for each size. Those sizes exercise the empty case, each group modulus, the boundary either side of ten, and the by-10s, by-100s and by-1000s rollups that build children.

ty goes from 35 diagnostics to 32, which is these three and nothing else. ruff format --check, ruff check and the unit suite (366 passing) are unchanged.

Why this one and not the rest

Of the remaining type-checker findings this is the only one that is a single decision about a single structure. The others each need their own judgment, mostly about where ty cannot see through self.__class__ in pages/page.py or a widened dict in api/client.py. None of them is a runtime bug, checked individually, and none is in this pull request.

[ciq-it-service-account]

✅ Snyk checks have passed. No issues have been found so far.

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