Fix Chat Customizations layout and context (#333281)

* Fix Chat Customizations layout and context

Preserve the originating chat session when opening Customizations, align Tools and migration content, keep migration controls clear of scrollbars, and improve MCP status contrast.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix migration control clipping

Keep migration content at its original width and prevent the Select all label from clipping at the content edge.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Update Chat Customizations screenshots

Accept the reviewed Tools and migration fixture baselines from CI.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>