fix: resolve MyPy type check failures

[Aaron ("AJ") Steers (aaronsteers)]

Summary\n\nFixes 10 MyPy type check errors that fail in CI but not locally (likely due to CI using Python 3.10 + Poetry 1.8.4 with different dependency resolution):\n\n- checkpoint_reader.py: self.current_slice typed as StreamSlice | None — use local current variable after _find_next_slice() to avoid repeated property access through Optional type\n- utils.py / connector_builder_handler.py: Mapping.get() returns Any — wrap with bool() to satisfy -> bool return type\n- default_file_based_stream.py: cursor setter override type mismatch — add # type: ignore[override] (matching existing ignore on the getter)\n\nRequested by: Aaron ("AJ") Steers (@aaronsteers)

Link to Devin session: https://app.devin.ai/sessions/9f900ef4021147adb369727617d08827

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PyTest Results (Fast)

4 094 tests  ±0   4 083 ✅ ±0   8m 17s ⏱️ -9s
    1 suites ±0      11 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit 66dd637. ± Comparison against base commit a7b0929.

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PyTest Results (Full)

4 097 tests  ±0   4 085 ✅ ±0   11m 31s ⏱️ -6s
    1 suites ±0      12 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit 66dd637. ± Comparison against base commit a7b0929.