fix(bigtable): correct get_bigtable_data_client return annotation

[anxkhn]

Please ensure you have read the contribution guide before creating a pull request.

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

N/A (no existing issue; described below per CONTRIBUTING for a small fix).

2. Or, if no issue exists, describe the change:

Problem:

In src/google/adk/tools/bigtable/client.py, get_bigtable_data_client is
annotated as returning bigtable.BigtableDataClient:

from google.cloud import bigtable
from google.cloud.bigtable import data

def get_bigtable_data_client(
    *, project: str, credentials: Credentials
) -> bigtable.BigtableDataClient:      # <- name not exported here
    bigtable_data_client = data.BigtableDataClient(  # <- actual type
        project=project, credentials=credentials, client_info=_get_client_info()
    )
    return bigtable_data_client

BigtableDataClient is not exported from the google.cloud.bigtable top-level
namespace; that package only exports Client:

>>> import google.cloud.bigtable as bt
>>> bt.__all__
['__version__', 'Client']
>>> hasattr(bt, 'BigtableDataClient')
False

The type actually constructed and returned is google.cloud.bigtable.data.BigtableDataClient,
and data is already imported in the module. Because the module uses
from __future__ import annotations, the annotation is a lazy string, so this
does not raise at import time. But it is a wrong type reference: any tooling that
resolves the annotation fails. For example:

>>> import typing
>>> typing.get_type_hints(get_bigtable_data_client)
AttributeError: module 'google.cloud.bigtable' has no attribute 'BigtableDataClient'

This breaks typing.get_type_hints(), and by extension type checkers and API-doc
generators that resolve return annotations. The sibling get_bigtable_admin_client
in the same file is correctly annotated -> bigtable.Client.

Solution:

Point the annotation at the type that is actually constructed and returned,
data.BigtableDataClient (import already present). One-character-scope change; no
behavior change. The from google.cloud import bigtable import is kept because
get_bigtable_admin_client still uses bigtable.Client.

def get_bigtable_data_client(
    *, project: str, credentials: Credentials
) -> data.BigtableDataClient:

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Added test_get_bigtable_data_client_return_annotation_resolves in
tests/unittests/tools/bigtable/test_client.py, asserting the return annotation
now resolves to the real type:

def test_get_bigtable_data_client_return_annotation_resolves():
  """The data client return annotation must resolve to a real type."""
  hints = typing.get_type_hints(client.get_bigtable_data_client)
  assert hints["return"] is data.BigtableDataClient

The test is a genuine regression guard: with the old annotation it fails with
AttributeError: module 'google.cloud.bigtable' has no attribute 'BigtableDataClient';
with the fix it passes.

pytest results:

$ pytest tests/unittests/tools/bigtable/test_client.py -q
...                                                                      [100%]
3 passed

$ pytest tests/unittests/tools/bigtable/ -q
41 passed

Manual End-to-End (E2E) Tests:

Not applicable: this is a type-annotation-only correction with no runtime behavior
change (the returned object is unchanged). Resolution of the annotation is
demonstrated above and covered by the new unit test.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas. (N/A: one-line annotation fix, no comment needed.)
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end. (N/A: no runtime behavior change.)
• Any dependent changes have been merged and published in downstream modules. (N/A.)

Additional context

Precedent for this class of type-hint fix being accepted in this repo: maintainer
commit 9865e2bc ("fix(live): fix the return type hint of handle_function_calls_live"),
plus 1ce043a2 and f592de4c.