Preserve history session boundaries across list mutations - #1795

Removing or inserting entries before session_start_index currently leaves it pointing at the wrong place, so unbounded history ranges and searches omit current-session commands or include persisted ones.

Adjust the existing boundary after successful list mutations, including slices, pop() and remove(). When a replacement spans the boundary, recognize the leading retained persisted entries by identity. In-place filtering then agrees with deleting the same entries, and equal commands from different sessions are kept distinct. Self-assignment remains stable; in-place repetition that clears history resets the boundary. truncate() uses the shared deletion handling, and the public API fix is recorded under 4.4.0 in the changelog.

Validation on macOS:

• All 900 history tests pass, including 78 mutation/control cases covering filtering, negative/stepped slices, reordered and repeated entries, replacement iterators, failure-state preservation and composed operations. On pre-revision head a77a1a5b, 21 revised/new cases fail.
• Required make check, make test and strict make docs-test pass on Python 3.12: 2,914 tests passed and five skipped, with 100% statement coverage for history.py (99.4% overall).
• The exact test-group CI recipe also passes on Python 3.11, 3.13, 3.14, 3.15 and free-threaded 3.15: 2,913–2,915 passes with four to six skips per runtime. Workers were capped at two; the free-threaded interpreter reports the GIL disabled.
• make check and strict docs also pass on the CI quality/docs Python 3.15 runtime. Wheel and source distribution builds pass.

Linux and Windows remain for CI.

Closes #1788.