Qt-removal R7.3: port 3 confirmed test-coverage gaps into backend/tests/

[dovvnloading]

Summary

Ports the 3 test-coverage gaps identified during the R7 scoping recon: Qt-free survivor modules (graphlink_chart_data.py/graphlink_chart_agent.py, api_provider.py, graphlink_licensing.py) that backend/ imports and uses in live production paths, but whose own validation/caching/schema logic had zero backend/tests coverage.

What Changed

• backend/tests/test_chart_data_validation.py (new, 4 tests) — canonicalize_chart_data's own ChartDataError branch (non-finite numbers, wrong container types, sankey-cycle rejection, duplicate-flow aggregation) and ChartDataAgent.validate_chart_data's delegation + in-place canonicalization contract.
• backend/tests/test_ollama_capability_cache.py (new, 6 tests) — api_provider's Ollama capability cache invalidation rules, ported from the legacy suite plus one new test proving the cache actually caches.
• backend/tests/test_settings.py (+2 tests) — SettingsManager.schema_version fresh-file default and old-file backfill-on-load.

Each file's docstring documents what was deliberately not ported: the Qt-tainted half of each legacy test (ModelPullWorkerThread, ChatScene/SceneSerializer chat-payload schema versioning) has no backend/ equivalent yet, so porting those would test nothing real.

Why

• These are live production code paths (canvas.py:32, agents.py:75, agents.py:72, response_parsing.py:25), not legacy-only code — they needed real coverage before the R7.6 cutover deletes the legacy Qt app and its tests.
• Every new test was mutation-tested against the actual production code (deliberately breaking the logic under test and confirming the test fails) before being counted as real coverage.

Validation

• python -m compileall -q . passes (repo root)
• tests/test_no_qt_anywhere.py — burn-down pin unchanged at 152/84/68 (test-only change, zero Qt files touched)
• Full backend suite: 2387 passed (2375 baseline + 12 new), no regressions
• Frontend sanity: 1053 vitest passed, vite build clean

Additional Context

Part of the Qt-removal parity/cutover phase (R7). R7.1 (prereq fixes, #139) and R7.2 (relocate 33 survivor modules, #140) are merged; this is R7.3. Next up: R7.4 (build the deferred Ollama/Llama.cpp/API-provider settings pages).