[SYNPY-1875] Copy and deprecate copyWiki function

[andrewelamb]

Problem:

synapseutils.copyWiki() was the only way to copy a wiki tree from one entity to
another, and it is legacy code that sits outside the object-oriented model layer:

• It is a free-standing, sync-only function that takes a syn client as its first
  positional argument, so it cannot be used from an async context and does not follow
  the synapse_client keyword pattern used everywhere else in the new API.
• It uses camelCase arguments (entitySubPageId, destinationSubPageId,
  updateLinks, entityMap) that are inconsistent with the rest of the OOP models.
• It has no counterpart on the WikiPage model, so users who have adopted
  synapseclient.models had to drop back down to synapseutils to copy a wiki —
  the same gap that was closed for DownloadList, sync_to_synapse /
  sync_from_synapse, storage locations, and migration in earlier work.

This is the copyWiki half of the ongoing migration of synapseutils bulk
operations onto the object-oriented models (SYNPY-1875).

Solution:

Add WikiPage.copy_async() (and its generated sync WikiPage.copy()) as the
object-oriented replacement for copyWiki(), and deprecate the legacy function.

New API function — synapseclient/api/file_services.py

Added post_file_handles_copy(), wrapping POST /filehandles/copy, used to copy
wiki attachment file handles to the destination. It batches requests by
MAX_FILE_HANDLE_PER_COPY_REQUEST internally so callers do not have to, and it is
re-exported from synapseclient.api. This puts the file-handle-copy REST call in the
api/ layer where the model can reuse it, rather than reimplementing the batching
that synapseutils.copyFileHandles() does.

Deprecation — synapseutils/copy_functions.py

copyWiki() is marked @deprecated(version="5.0.0") for removal in 6.0.0, with a
docstring migration example pointing at WikiPage.copy / WikiPage.copy_async. The
function itself is unchanged, so existing callers keep working.

Supporting changes

• synapseclient/models/protocols/wikipage_protocol.py — sync copy() signature
  added so IDE type hints resolve.
• docs/reference/experimental/sync/wiki.md and
  docs/reference/experimental/async/wiki.md — copy / copy_async added to the
  WikiPage members lists so mkdocstrings generates the anchors and
  [synapseclient.models.WikiPage.copy] cross-references resolve.

The orchestration is split into small module-level helpers
(_validate_and_format_copy_inputs, _copy_wiki_pages,
_collect_wiki_sub_tree_headers, _update_internal_links,
_update_synapse_id_references, _ensure_destination_has_no_root_wiki,
_get_existing_destination_wiki_page) so each piece is unit-testable without
standing up a full wiki tree.

Testing:

Unit tests — tests/unit/synapseclient/models/async/unit_test_wiki_async.py

Added a TestWikiPageCopy class covering copy_async end to end with mocks, plus a
test class per helper. Coverage includes:

• Validation failures: missing owner_id, invalid destination_owner_id, invalid
  entity_map keys and values, non-numeric sub-page IDs.
• Source edge cases: source entity with no wiki returns an empty list, unknown
  entity_sub_page_id raises ValueError, sub-page ID given for a source with no
  wiki raises ValueError, sub-page ID type coercion.
• Destination edge cases: destination that already has a root wiki without
  destination_sub_page_id, nonexistent destination_sub_page_id, and error
  propagation from the destination-page and header-tree lookups.
• The four rewrite combinations: update_links only (the default), entity_map
  only, both, and neither.
• Helper behavior: sub-tree collection ordering (parents before children) and
  non-mutation of the input headers, link and Synapse-ID rewriting across every
  copied page, None markdown becoming an empty string, and hierarchy preservation
  in _copy_wiki_pages including the create-under and write-into destination cases.

Integration tests — tests/integration/synapseclient/models/async/test_wiki_async.py

Added a TestWikiPageCopy class that copies against a real Synapse backend:

• test_copy_entire_wiki_tree — full tree copy, verifying hierarchy, markdown,
  attachments, and rewritten internal links.
• test_copy_wiki_sub_tree — copying only the sub-tree rooted at a given page.
• test_copy_wiki_into_existing_destination_page — copying into an existing
  destination wiki page via destination_sub_page_id.
• test_copy_wiki_from_entity_without_wiki — source with no wiki returns an empty
  list.