fix(session-picker): avoid MissingStyle when confirming session delete

[syf2211]

Summary

Fixes a crash when pressing d to delete a session in the session picker. The delete confirmation prompt now uses Textual markup (Content.from_markup) instead of Rich Text.append with $primary theme tokens.

Motivation

OptionList renders option prompts via Content.from_rich_text, which cannot parse Textual theme variables like $primary. The delete confirmation path used text.append("d", style=SHORTCUT_STYLE) where SHORTCUT_STYLE = "b not dim $primary", causing:

MissingStyle: Failed to get style 'b not dim $primary'

Reported in #907.

Changes

• Build delete confirmation prompt with Content.from_markup and shortcut('d'), matching the pattern used in help text and quit_manager.py
• Widen _option_text / _show_delete_state prompt types to Text | Content
• Add regression test that exercises OptionList._update_lines() with the confirmation prompt

Tests

python3 -m pytest tests/cli/textual_ui/test_session_picker.py -q -o addopts=""

Result: 33 passed

Notes

• Only the confirmation state is affected; feedback/pending prompts still use Rich Text without $primary
• Reviewed with composer-2.5: APPROVE

Fixes #907