Strip source-buffer properties from grabbed region context

[liaowang11]

agent-shell--get-region-context built its single-line file-link preview with buffer-substring, and agent-shell--get-numbered-region built its numbered preview the same way. buffer-substring copies all text properties, so when the source buffer was a markdown-mode or poly-markdown buffer, markdown-mode's font-lock leaked into the grabbed context: the emphasis markup characters carried 'invisible markdown-markup' + 'markdown-markup-face' and the enclosed text carried italic faces. _hello_world_ then showed up in the compose buffer as helloworld (underscores hidden, inner text italic) — the same intraword-underscore symptom the response renderer was already fixed to avoid.

Use buffer-substring-no-properties for both preview paths so the context carries plain text and no foreign-mode styling.

Checklist

• I agree to communicate (PR description and comments) with the author myself (not AI-generated).
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• I've filed a feature request/discussion for a new feature.
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• I've added tests where applicable.
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[xenodium]

Thanks for the PR!

Wouldn't we want to at least keep face and 'font-lock-face? Often gives us syntax highlighting for free.

[liaowang11]

Thanks for the PR!

Wouldn't we want to at least keep face and 'font-lock-face? Often gives us syntax highlighting for free.

Yes, it's a good idea to keep some faces, I added a custom buffer-substring function to keep both.

[xenodium]

Thanks for the PR and follow-up changes.