fix(tui): prevent sidebar selection copy bleed

[nayafyt]

Issue for this PR

Closes#41692

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This fixes a TUI text-selection bug where selecting conversation text could also include the right Context sidebar text in clipboard output.

Changes in this PR:

• Mark the sidebar subtree as non-selectable in the session sidebar implementation.
• Add a regression test to verify the subtree is recursively non-selectable.

Why this works:

• OpenTUI selection can traverse selectable siblings during drag.
• Making the sidebar subtree non-selectable removes sidebar text from selection candidates, so copied selection stays scoped to intended content.

How did you verify your code works?

• Ran: ./script/bun-docker.sh run --cwd packages/tui test test/util/sidebar-selection.test.ts
• Ran: ./script/bun-docker.sh run --cwd packages/tui typecheck
• Manual behavior check in TUI: selecting conversation text no longer copies Context sidebar text.

Screenshots / recordings

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

If you do not follow this template your PR will be automatically rejected.

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