Add input validation to Subject.set_color_depth - #382

🤖 Draft implementation — refinement loop (3a307af295a281a9ba70f6d2fa691c78)

Drafted automatically by the Snowplow refinement loop (refine → product review → implement → review) from a Notion refinement, and opened here for a human to finish.

Heads-up: this is a starting point. A human reviewer should verify, finish, and run CI before merging.

What the loop did

Add input validation to Subject.set_color_depth

Subject.set_color_depth(depth) accepted any integer, including zero and negatives, which are nonsensical screen colour-depth values. The sibling setters set_screen_resolution and set_viewport already guard their inputs with the repo's greater_than contract helper; set_color_depth was missing the same guard.

Changes

snowplow_tracker/subject.py

• Added greater_than(depth, 0) as the first statement of set_color_depth, raising ValueError for zero and negative inputs. Reuses the existing import on line 19 — no new error-handling patterns introduced.
• Updated the :type depth: docstring annotation from int to int,>0 to match sibling setters.

snowplow_tracker/test/unit/test_subject.py

• Added test_set_color_depth_validation to TestSubject, asserting ValueError for depth=0 and depth=-1, and no exception for depth=24.

Key decisions

• Reused the existing greater_than contract helper — consistent with set_screen_resolution and set_viewport, no new patterns.
• Scope held to set_color_depth only; no other setters touched.
• No wire-format or schema change — the cd field is unchanged; only invalid values are now rejected at the SDK boundary.

Review checklist

• greater_than(depth, 0) is the first statement in set_color_depth, before any assignment.
• The docstring annotation reads int,>0.
• test_set_color_depth_validation covers 0 and -1 (expect ValueError) and 24 (expect no exception).
• All existing tests pass (142 unit tests confirmed green).
• No changes outside the two files listed above.