fix(coding-agent): return to ask options after custom escape

[roboomp]

Repro

Running bun --cwd=packages/coding-agent test test/tools/ask.test.ts -t "aborts when editor is cancelled in single-question flow" reproduced the reported behavior: the existing test expected Other (type your own) followed by editor Escape to raise ToolAbortError("Ask tool was cancelled by the user").

Cause

packages/coding-agent/src/tools/ask.ts treated ui.editor(...) returning undefined as a completed empty answer path inside askSingleQuestion: single-question flows then aborted on empty selectedOptions/customInput, while multi-question flows auto-advanced with an empty result.

Fix

• Keep askSingleQuestion on the current question when the custom editor is dismissed, for both single-select and multi-select prompts.
• Preserve existing selections/custom input while re-showing the option list.
• Update ask-tool regression tests for single-question, multi-question, and multi-select Escape-from-custom flows.
• Add the coding-agent changelog entry for Can't return to options list in ask tool #3269.

Verification

bun --cwd=packages/coding-agent test test/tools/ask.test.ts passed: 35 tests, 155 assertions. gh_push_branch pre-publish gate passed before pushing. Fixes #3269