[ADD] ms365: read shared O365 documents (Graph Shares + text extraction)

[beorngb]

Extends the ms365 MCP plugin to read Office 365 documents shared with the
account — the case the existing tools couldn't reach (they only read files by
known SharePoint site_id + file_path or the user's own OneDrive).

New tools

• m365_read_shared(sharing_url | drive_id+item_id) — reads a shared document by
  share link (from email) or by an item from m365_list_shared. Resolves via the
  Graph Shares API (/shares/{shareId}/driveItem) or /drives/{id}/items/{id},
  then returns extracted text. Works cross-tenant with the recipient's own
  delegated token — no app in the sharer's tenant needed.
• m365_list_shared() — lists "Shared with me" (/me/drive/sharedWithMe),
  paginated (cap 200 items / 5 pages, truncated flag), returning
  drive_id/item_id to pass to m365_read_shared.

Also

• New plugins/ms365/extract.py: extract_text (docx/xlsx/pdf/text via
  python-docx/openpyxl/pypdf) and encode_sharing_url (Graph shareId).
• m365_read_file/m365_read_drive_file now extract Office text on binary
  content instead of returning "Binary file" — text files still decode as before
  (utf-8-first).
• Content read is robust: resolve item metadata → fetch the pre-authenticated
  @microsoft.graph.downloadUrl with no auth header (avoids leaking the Graph
  bearer to the storage host), streamed with a 50 MB size guard; folders and
  missing-downloadUrl items are guarded with clear errors.
• Deps declared in the manifest and the ms365 pyproject extra.

Only the MCP-server path (server.py) is touched; the legacy in-process tag path
is untouched.

Tests — 35 unit tests (tests/unit/plugins/ms365/): extraction (all formats

• empty/unsupported/corrupt/truncation/uppercase-ext), share-URL encoding,
  pagination, input contract, folder/size guards, error mapping. Repo-wide unit
  suite green.

Runtime prerequisite (not in this PR) — the plugin is currently unconfigured
(no Azure app client_id in its DB config), so this is not yet live-verifiable.
To enable: register/point the Azure AD app, grant the Files.Read.All delegated
permission with admin consent, re-authenticate the account(s) via the OAuth flow,
then confirm end-to-end.

Follow-ups (non-blocking)

• m365_read_shared does not update the "current file" continuity state that
  read_file/read_drive_file feed.
• context.py tool docs are on the legacy path and don't reach the live MCP
  system prompt (agents learn the tools from the Tool() schemas).
• openpyxl/pypdf/python-docx are duplicated between the data and ms365
  pyproject extras (harmless).