test: restore 100% coverage — cover the 46 snapshot.py branches left untested by #226

[pranavz28]

Root cause

The PER-9772 SDK-regression dispatch run (28640545663) failed make coverage with zero test failures — only the coverage gate tripped: percy/snapshot.py 564 stmts, 46 miss, 92% → TOTAL 94% < fail_under = 100 (.coveragerc).

This is not CLI-flavor dependent. The normal push-CI run for the same commit (28495875576 / 28495790591, released @percy/cli) fails with the byte-identical missing-lines list. Master has been red since #226 (feat: CORS iframes + closed shadow DOM parity) merged: it added new guard/error-fallback branches to percy/snapshot.py without tests for all of them.

Exact missing lines (identical in both the dispatch and push runs):

36-37, 247-248, 253-254, 272-274, 303-305, 316-317, 341-343, 364-366,
374, 377, 408-409, 416, 460-462, 483-484, 549, 557-560, 564-565, 605,
607-608, 613-614, 807-808, 959-963

Fix

Add deterministic, CLI-independent unit tests to tests/test_snapshot_units.py (runs as a plain unittest module, no percy exec) covering every missed branch:

Lines	Branch covered
36-37	invalid PERCY_RESIZE_SETTLE_SECONDS env falls back to 0.5 (module reload)
247-248, 253-254	_should_skip_iframe: unparseable origin; cross-origin frame missing data-percy-element-id
272-274	process_frame_tree: depth > max_frame_depth cap
303-305	iframe element no longer found by data-percy-element-id
316-317	post-switch document.URL read failure treated as unsupported
341-343	empty PercyDOM.serialize result for a frame
364-366	nested-iframe enumeration failure keeps parent capture
374, 377	skipped nested child (continue); successful nested capture collected
408-409	default_content() failure swallowed at depth 1
416	PercyContextLost chains the original processing error as __cause__
460-462	_capture_cors_iframes outer error guard (malformed enumeration entry)
483-484	driver.capabilities access failure treated as non-Chromium
549, 557-560, 564-565	closed-shadow-root CDP: missing objectIds, DOM.resolveNode failure, DOM.getDocument + DOM.disable failures
605, 607-608, 613-614	get_origin no-scheme/netloc and parse-raise paths; _get_origin compat shim
807-808	driver.current_url read failure in get_serialized_dom
959-963	PERCY_DEBUG responsive snapshot dump (write path + failure path)

No production code changes and no # pragma: no cover — all branches are deterministically exercisable with mocks. Since the new tests never touch the CLI, coverage of these lines no longer depends on which @percy/cli (released or built-from-git) is installed.

Verification

Local, Python 3.9.18, mirroring the Makefile coverage target exactly (tests.test_snapshot under percy exec --testing with @percy/cli 1.32.2 + the five plain modules, coverage combine, coverage report):

percy/snapshot.py            564      0   100%
TOTAL                        759      0   100%

• All 33 tests in tests.test_snapshot_units pass (was 8, +25 new); tests.test_snapshot unchanged and green (96 tests).
• pylint percy/* tests/* → 10.00/10.
• Both failing CI runs (dispatch with git-built CLI, push with released CLI) missed the exact same 46 lines, and everything else was already covered under both CLI flavors — so covering these lines with CLI-independent unit tests makes the gate pass on both the normal CI path and the PER-9772 regression-dispatch path.

🤖 Generated with Claude Code