feat: Rerouted ReadRows to data client

[daniel-sanche]

Migrating over @gkevinzheng PR from bigtable monorepo googleapis/python-bigtable#1308

Original description:

Changes Made:

• Added methods to convert Row and Cell objects in the data client to PartialRowData and Cell objects in the legacy client.
• Removed legacy client code related to processing ReadRowResponse chunks and testing ReadRowResponse chunks.
• Removed _update_message_request from RowSet because it's no longer needed to create a ReadRowQuery
• Rerouted read_row and read_rows to use their data client counterparts in table.py.

Additional Changes:

Applied some guards against removed semi-private APIs, after internal discussion:

• 6941199: we had a couple technically public, but not meant to be accessed attributes. This commit replaces them with properties that raise a warning and return None, rather than completely removing them
• c9617ac: Similarily, the PartialRowsData constructor was technically public, but not intended to be used. This change returns an empty generator when the previous constructor is used, with a warning
• b51292e: handles a bug that could occur if None is passed for retries
• 92da437: added better typing to generator

Note to reviewers: This PR has already been reviewed and merged to a staging branch, with the intention of doing a single merge to main. We are now planning to slowly rollout these changes back to the main branch. Minimal re-review should be necessary

[gemini-code-assist]

Code Review

This pull request refactors the Bigtable client to delegate row reading and streaming to the underlying _table_impl data client, deprecating legacy classes like _RowMerger and several attributes on PartialRowsData. The review feedback identifies potential AttributeError risks when accessing retry.deadline directly (since google.api_core.retry.Retry typically uses _deadline internally) in table.py and unit tests, as well as when calling close() on _generator in row_data.py if the generator does not support it.