Add results yank option to copy INSERT statements for all visible rows

[koianaoki]

Summary

• Added a new results yank action yI to copy INSERT statements for all currently visible rows.
• Kept existing yi behavior for copying a single selected row INSERT statement.
• Implemented shared INSERT statement builder logic in the results mixin to avoid duplication.
• Updated help text and leader menu labels to clarify row vs all-rows behavior.

Behavior details

• yi: copies one INSERT statement for the selected row.
• yI: copies multiple INSERT statements (one per row) for the currently visible rows.
• When results are filtered, yI uses only the filtered matching rows.

Files changed

• sqlit/domains/results/ui/mixins/results.py
• sqlit/core/keymap.py
• sqlit/domains/shell/state/machine.py
• tests/unit/test_results_yank_insert.py

Testing

• Static updates only in this environment (no test execution performed).

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Codex Task