[spanner] RequestOptions.transaction_tag is unexpectedly mutated to '' when passed to snapshot.read()

### Determine this is the right repository

- [x] I determined this is the correct repository in which to report this bug.

### Summary of the issue

When passing a RequestOptions instance with a transaction_tag to snapshot.read(), the transaction_tag attribute of the original object is unexpectedly mutated to '' (empty string).

### API client name and version

google-cloud-spanner==3.69.0

### Reproduction steps: code

from google.cloud import spanner
from google.cloud.spanner_v1.types import RequestOptions

spanner_client = spanner.Client()
instance = spanner_client.instance("your-instance-id")
database = instance.database("your-database-id")

request_opts = RequestOptions(transaction_tag="tran1")

print("Before read:", request_opts.transaction_tag)

with database.snapshot() as snapshot:
snapshot.read(
table="Singers",
columns=("SingerId", "FirstName"),
keyset=spanner.KeySet(all_=True),
request_options=request_opts,
)

print("After read:", request_opts.transaction_tag)

### Reproduction steps: supporting files

_No response_

### Reproduction steps: actual results

Before read: tran1
After read:

### Reproduction steps: expected results

Before read: tran1
After read: tran1

### OS & version + platform

Windows 11

### Python environment

Python 3.14.6

### Python dependencies

pip list
Package Version
---------------------------------- ---------
certifi 2026.7.22
cffi 2.1.0
charset-normalizer 3.4.9
cryptography 49.0.0
google-api-core 2.33.0
google-auth 2.56.2
google-cloud-core 2.6.0
google-cloud-monitoring 2.31.0
google-cloud-spanner 3.69.0
google-cloud-storage 3.13.0
google-crc32c 1.8.0
google-resumable-media 2.10.0
googleapis-common-protos 1.75.0
grpc-google-iam-v1 0.14.4
grpc-interceptor 0.15.4
grpcio 1.82.1
grpcio-status 1.82.1
idna 3.18
mmh3 5.2.1
opentelemetry-api 1.44.0
opentelemetry-resourcedetector-gcp 1.12.0a0
opentelemetry-sdk 1.44.0
opentelemetry-semantic-conventions 0.65b0
pip 26.1.2
proto-plus 1.28.2
protobuf 7.35.1
pyasn1 0.6.4
pyasn1_modules 0.4.2
pycparser 3.0
requests 2.34.2
sqlparse 0.5.5
typing_extensions 4.16.0
urllib3 2.7.0

### Additional context

Passing an object as an argument should not destructively mutate the caller's input object.