Tier 2 of #1725 (first item).

Problem

SphinxNeedsData stashes all of its data as private attributes on the Sphinx BuildEnvironment (e.g. env._needs_all_needs, env._needs_view, env._needs_schema, …) and _needs_services on the Sphinx app. mypy can't see these, so data.py carried an override masking the fallout:

[[tool.mypy.overrides]]
module = ["sphinx_needs.data"]
disable_error_code = ["attr-defined", "no-any-return"]

That override hid 45 errors (9 dynamic env attributes + 1 app attribute, each generating an attr-defined and, where returned, a no-any-return).

Fix

Declare the stashed attributes on TYPE_CHECKING-only subclasses — _NeedsEnv(BuildEnvironment) and _NeedsApp(Sphinx) — and cast self.env to _NeedsEnv once in __init__. Every accessor body is left untouched.

• The two helper classes exist only under TYPE_CHECKING, so there's no runtime footprint.
• cast("_NeedsEnv", env) is a runtime no-op — it returns env unchanged.
• _NeedsEnv is-a BuildEnvironment, so nothing that consumes .env is affected.

This removes the last per-module disable_error_code override from the mypy config — data.py is now type-checked under full --strict with no suppressions.

Verification

• mypy --strict — clean (78 files); all 45 previously-suppressed errors resolved with the override deleted
• ruff check / ruff format — clean
• 56 tests pass across test_basic_doc, test_api_usage, test_need_item_api, test_needpie_with_zero_needs, test_service_github, test_needs_builder (exercise need creation, views, services, umls, and the github app-attribute path)

No changelog entry: internal type-checking change, no runtime behaviour difference (consistent with the Tier 1 PR #1726).

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https://claude.ai/code/session_016hgF7RDxmXvBnFhCkP3sNh

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