Add copy and export for the Ask Optiq conversation

[dhingora-amd]

Motivation

The Ask Optiq assistant had no way to get a conversation out of the panel.
This adds Copy and Export so users can save an investigation, share findings,
or paste a transcript elsewhere.

Technical Details

• Adds Copy and Export icon buttons to the assistant composer, beside
  Clear; both stay disabled until there is a conversation to save.
• Copy places the transcript on the clipboard (ImGui::SetClipboardText).
• Export writes it to a Markdown/text file via AppWindow::ShowSaveFileDialog.
• New BuildTranscriptText() renders the transcript as Markdown (You / Optiq
  turns, notices as blockquotes); charts are noted rather than snapshotted
  since they redraw from live model data.
• Reuses NotificationManager for success/failure feedback — no new logging
  or file-write helpers introduced.
• Files: src/view/src/agenticprofiling/rocprofvis_ai_assistant.{h,_render.cpp}.

[tomk-amd]

None of these are blockers, but recording them here...

Issues

• Medium (UX) — rocprofvis_ai_assistant_render.cpp (Export IconButton):
  Export uses ICON_ARROW_IN_BOX, which is already the import icon ("Import from Basic" in rocprofvis_multi_track_table.cpp). Tooltip says export, but the glyph means the opposite. Prefer ICON_DOCUMENT, ICON_ARCHIVE, or ICON_ARROW_FORWARD unless a dedicated export glyph is added.
• Low — ExportTranscript() write path:
  After is_open(), success is toasted without checking write/flush. A full disk or later write error still looks like success. Same gap exists in ScriptEditor::WriteFile; worth fixing here if you touch this path.

Nits

• Copy/export I/O lives in _render.cpp, whose file comment says that file only draws ImGui and that turn/state machinery stays in rocprofvis_ai_assistant.cpp. Functionally fine (assistant.cpp already includes AppWindow); a later split could move these three methods.
• Native save dialogs append the first filter’s extension when the name has none (AppWindow around the NFD path). Markdown is first, so a “Text” save named notes becomes notes.md. Pre-existing dialog behavior; not introduced here.
• Header comment says BuildTranscriptText() “renders” the transcript; it serializes it.