Do not advertise kernel subshell support

[cxzhong]

Description

Sage now removes kernel subshells from the features advertised in its
kernel_info_reply.

With ipykernel 7, advertising this feature allows JupyterLab to route widget
comm messages to a kernel subshell running in another OS thread. Sage libraries
are not generally safe to enter from arbitrary threads; in particular, the
@interact example in the issue reaches cypari2/PARI from that thread and
segfaults after the initial evaluation.

Filtering the unsupported feature keeps these callbacks on the main kernel
shell while preserving the other features reported by ipykernel.

Fixes #42567.

Validation

• ./sage -t --force-lib src/sage/repl/ipython_kernel/kernel.py (16 tests passed)
• Started a temporary Sage kernel and verified that its advertised features
  contain debugger but not kernel subshells
• git diff --check

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

Documentation is unchanged because this only corrects kernel feature
negotiation.

⌛ Dependencies

None.

[github-actions]

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