PingingPool.bind() and FixedSizePool.bind() do not pass labels to BatchCreateSessions session_template

## Environment

- **OS:** Linux / macOS
- **Python:** 3.10+
- **google-cloud-spanner:** 3.55.0 (also confirmed on `main` branch as of 2026-02-11)

## Summary

`PingingPool.bind()` and `FixedSizePool.bind()` do not include user-assigned `labels` in the `BatchCreateSessionsRequest` `session_template`. This means sessions created eagerly during `bind()` never receive the labels passed to the pool constructor, even though the `labels` parameter is accepted and documented.

## Expected behavior

Sessions created during `PingingPool.bind()` should carry the labels passed via `PingingPool(labels={"key": "value"})`, matching the behavior of `_new_session()` which correctly passes labels.

## Actual behavior

The `session_template` in `bind()` only includes `creator_role`:

```python
# pool.py - PingingPool.bind()
request = BatchCreateSessionsRequest(
database=database.name,
session_count=self.size,
session_template=Session(creator_role=self.database_role), # labels missing
)
```

While `_new_session()` correctly passes labels:

```python
# pool.py - AbstractSessionPool._new_session()
return self._database.session(
labels=self.labels, database_role=self.database_role
)
```

As a result, only sessions lazily created via `_new_session()` (e.g., to replace expired sessions) receive labels. The initial batch of sessions created in `bind()` does not.

## Steps to reproduce

```python
from google.cloud import spanner
from google.cloud.spanner_v1 import SpannerClient

pool = spanner.PingingPool(size=2, default_timeout=10, labels={"env": "test"})
client = spanner.Client(project="my-project")
instance = client.instance("my-instance")
db = instance.database("my-database", pool=pool)

# Check sessions via Admin API
admin_client = SpannerClient()
db_path = f"projects/my-project/instances/my-instance/databases/my-database"
for session in admin_client.list_sessions(database=db_path):
print(dict(session.labels))
# Prints: {} (empty) -- expected: {"env": "test"}

pool.clear()
```

## Suggested fix

Include `self._labels` (or `self.labels`) in the `session_template` passed to `BatchCreateSessionsRequest` in both `PingingPool.bind()` and `FixedSizePool.bind()`:

```python
request = BatchCreateSessionsRequest(
database=database.name,
session_count=self.size,
session_template=Session(
creator_role=self.database_role,
labels=self._labels, # <-- add this
),
)
```

The Spanner API's `BatchCreateSessionsRequest.session_template` already supports `labels` — we confirmed that passing labels in the template correctly applies them to all created sessions.