[Outlook] Use selected email text as input instead of full body - #2039

Summary

Closes #1448.

When a user highlights text inside an email, the Outlook add-in now uses just the selection as the message context instead of always injecting the full email body.

• outlookMailContext.js: adds getSelectedDataAsync(item) (soft-fails to null, never rejects) and includes selectedText in the mail-context snapshot. Adds fetchCurrentSelectedText() for a fresh, pre-send re-read — Office.js has no "selection changed" event, so a snapshot taken on mount can go stale the moment the user changes their highlight.
• buildChatApiMessages.js: combineUserTextWithEmailContext now accepts currentSelectedText and prefers it over currentBodyText when non-empty; falls back to the body otherwise (unchanged behavior when there's no selection).
• useOutlookMailContextSnapshot.js: adds a useSelection toggle (defaults to true, resets on email change like includeBody). buildSnapshotOverride() is now async and re-reads the live selection right before send.
• useOfficeChatAdapter.js: threads ctx.selectedText through to combineUserTextWithEmailContext.
• OfficeMailContextBanner.jsx / OfficeContextStrip.jsx: shows "Using selected text (N chars)" with a one-click "Use full email instead" toggle (and the reverse), and updates the collapsed summary line.
• OfficeChatPanel.jsx: forwards the new toggle and includes selection text in the live token-count estimate.

Scope: mail-only (matches the issue's acceptance criteria) — pinned/multi-selected emails and appointment (calendar) items are unaffected and always use their full body.

Acceptance criteria

• Highlighting text in the email and opening the add-in produces a prompt that uses only the selection.
• With no selection, behavior is identical to today (full body used).
• The user can override the default per message (one-click toggle in the context banner).
• Selection text is refreshed when the user re-runs (re-read fresh right before send).
• Works in read mode; compose-mode behavior is unchanged (falls back gracefully — getSelectedDataAsync not present → null).

Test plan

• npx jest --config tests/config/jest.config.js tests/unit/client — 153/153 passing, including new/updated coverage in outlook-mail-context.test.jsx, office-build-chat-api-messages.test.jsx, and use-outlook-mail-context-snapshot.test.jsx.
• npx eslint on all changed files — no errors (pre-existing unrelated warnings only).
• npx prettier --write on all changed files — no changes needed.
• Manual verification in an actual Outlook taskpane (not possible in this environment) — please confirm the "Use selection" / "Use full email instead" toggle behaves as expected in a live Outlook host, including compose mode.

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