Fix autocomplete crash from stale cursor locations

[koianaoki]

Motivation

• Prevent crashes when debounced autocomplete callbacks run with a stale TextArea.cursor_location that can contain a row beyond the current document, which previously could index past available lines.
• Ensure conversions between (row, col) and offset always produce values clamped to the current document bounds.

Description

• Update _location_to_offset to clamp stale or out-of-range row and col values to the current text content so it cannot index past available lines.
• Clamp offset inside _offset_to_location to the valid 0..len(text) range before computing (row, col).
• Add regression tests in tests/unit/test_autocomplete_location.py that cover a stale row beyond the last line, an overlong column, and out-of-range offsets.

Testing

• Ran pytest tests/unit/test_autocomplete_location.py -q in the environment where textual was not installed and collection failed with ModuleNotFoundError (expected in that environment).
• Ran uv run pytest tests/unit/test_autocomplete_location.py -q which executed the new tests and reported 3 passed.
• Ran uv run ruff check sqlit/domains/query/ui/mixins/autocomplete.py tests/unit/test_autocomplete_location.py which completed with no linting issues.

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Codex Task

[koianaoki]

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