ci: Add mypy to ci and update types in code

[FBumann]

Summary

Adds mypy type checking to CI and fixes all type errors to pass strict mode.

Changes

CI (ci.yml):

• Added mypy type check step after format check

Type fixes:

• figures.py: Added proper type parameters for Iterator, list[go.Frame] return types, and dict[str, Any] for kwargs
• config.py: Used cast() with quoted type strings for restoring options
• common.py: Added type parameter dict[str, object] for attrs argument
• plotting.py: Changed np.ndarray to npt.ArrayLike for array-like input
• __init__.py: Added # type: ignore[no-untyped-call] for xarray's untyped accessor decorators

Test plan

• mypy xarray_plotly passes with no errors
• ruff check . passes
• pytest passes (121 tests)

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📝 Walkthrough

Walkthrough

This PR introduces mypy static type checking to the CI workflow and refines type hints throughout the codebase. Changes include narrowing parameter types, adding cast calls for type safety, and clarifying return types for multiple functions, without altering runtime behavior.

Changes

Cohort / File(s)	Summary
CI Type Checking .github/workflows/ci.yml	Adds mypy type check step running on xarray_plotly target, positioned after Format check and before Test step.
Type Annotation Refinement xarray_plotly/__init__.py	Adds type: ignore annotations on two Plotly accessor registration calls to suppress type checker warnings for untyped external library calls.
Parameter Type Narrowing xarray_plotly/common.py	Narrows _get_label_from_attrs parameter type from generic dict to dict[str, object] for stricter type checking.
Type Safety with Casts xarray_plotly/config.py	Imports cast and applies type casts to restored option values in set_options before reassignment for six configuration parameters.
Return Type Clarification xarray_plotly/figures.py	Expands type hints with Any import; clarifies return types for _iter_all_traces (to Iterator[Any]), _merge_frames and _merge_secondary_y_frames (to list[go.Frame]); refines update_traces signature with generic parameter types.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related PRs

• feat: add _iter_all_traces helper and rewrite manipulation notebook #18 — Modifies _iter_all_traces and update_traces in figures.py, overlapping with the type hint refinements in this PR.
• feat: add figure combination utilities (overlay, add_secondary_y, update_traces) #16 — Updates figures.update_traces function implementation, directly related to the type signature changes here.

Poem

🐰 Static types, oh what a sight!
Mypy checks, making code tight,
Casts and hints, all in a row,
Runtime safe, now we know! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The pull request title accurately summarizes the main changes: adding mypy type checking to CI and updating type annotations throughout the codebase.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

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Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@xarray_plotly/config.py`:
- Line 11: The failing TC006 rule is caused by unquoted type expressions passed
to cast() in xarray_plotly/config.py; locate the cast(...) calls that currently
use bool, str, and dict[str, tuple[str, ...]] and replace their type arguments
with string-literal types (e.g., cast("bool", ...), cast("str", ...),
cast("dict[str, tuple[str, ...]]", ...)) so the type expressions are quoted;
ensure you update each cast invocation (search for cast usages around the config
parsing/assignment logic) to use the quoted form.