Make mypy clean without --ignore-missing-imports (green CI on main)

[dantzert]

CI runs mypy . --ignore-missing-imports (.github/workflows/ci.yml:35). On committed main this is red: mypy 1.19 reports import-untyped for requests in testing_data/pull_usgs_delaware.py, and --ignore-missing-imports does not suppress import-untyped (verified: mypy . --ignore-missing-imports → Found 1 error in 1 file).

Per the maintainer's stated direction, the goal is to drop the --ignore-missing-imports flag and keep the whole tree clean.

Proposal:

• Make testing_data/pull_usgs_delaware.py clean on its own (add types-requests to dev/optional deps, or a [[tool.mypy.overrides]] with module = "requests" / scope it), then drop --ignore-missing-imports from the CI command.
• Narrow the lint step to the package + tests if testing_data/ is meant to be an optional standalone fetcher script (it currently isn't installed/tracked by packaging).
• Add a regression assertion that mypy modpods is clean.

Also a heads-up for the pending subsample work: while verifying, the in-flight changes currently on the working tree add 2 new mypy errors in modpods/model.py (the transform_dependent constraint block does initial_guess[idx, idx] / constraint_lhs[idx, …] where idx is an ndarray from the subsampler) plus a duplicate test_delay_io_train_camels_returns_model definition (no-redef). These should be resolved before that PR lands.

• Cost: Trivial (config tweak + stubs).
• Benefit: Medium (green CI; strict typing as a foundation for the typed API in follow scikit-learn or similar conventions #17).