Fix Gemini replay of unchanged CAD state

[Anteriousis]

Gemini previously attached the complete VibeScript state after every tool call, including reads that left it unchanged. This two-line fix passes the pre-tool context to the existing comparison helper, so unchanged state is omitted and changed state is delivered once.

Regression tests cover document revision, surface and workbench changes, native-engine state omission, and multiple calls within one model response. Tool results, call IDs, and Gemini thought signatures are preserved.

Verification

• For code changes, a test failed before the implementation and passes afterward; for non-code changes, the PR explains why TDD does not apply.
• The PR lists the exact build and test commands and their results.

Tests ran on Windows using the bundled Python interpreter and isolated pytest/jsonschema dependencies. In the commands below, $python and $git stand for the absolute executable paths used during verification; machine-specific paths are omitted.

Before the implementation:

& $python -m pytest -q src/Mod/VibeCAD/vibecad_tests/test_gemini_provider.py -k only_sends_changed_state --tb=short

Result: 3 failed, 1 passed, 9 deselected. The three VibeScript cases reproduced redundant state; the native-engine case already passed.

After the implementation:

& $python -m pytest -q src/Mod/VibeCAD/vibecad_tests/test_gemini_provider.py src/Mod/VibeCAD/vibecad_tests/test_provider_subprocess.py --tb=short

Result: 64 passed, 1 skipped.

Additional checks:

& $python -m py_compile src/Mod/VibeCAD/VibeCADProvider.py src/Mod/VibeCAD/vibecad_tests/test_gemini_provider.py
& $git diff --check

Both passed.

No paid model requests were made. Provider streams and the CAD bridge were mocked. No full C++ build or live GUI test was run; the production change is limited to Python request construction.

Issues

Fixes #175.

Follow-up to the provider payload optimization in #130.

Before and After Images

Not applicable: no GUI changes. Previously each unchanged read added another state block to the model conversation; afterward unchanged reads preserve the useful tool result without that redundant block.

Compatibility

• Existing public functions and APIs remain present.
• No preference keys, tool names, or schema fields were renamed or removed.
• Existing model settings and productive tool execution remain unchanged.
• Changed CAD state, tool results, call IDs, and thought signatures remain available.
• No deprecations, dependency changes, packaging changes, or public API breaks.