Restart gNMI after updating test authentication

[ashutosh-agrawal]

Description of PR

Summary:

Update the IPv6 management telemetry test to restart the gNMI service after
changing its temporary authentication configuration. The gNMI launch arguments
are generated when the service starts, so reloading supervisord does not
reliably apply a changed user_auth value.

The test now preserves and restores the original user_auth configuration and
waits for both the service and its TCP listener before sending the request. The
shared streaming telemetry context also restores client_auth when a test
raises an exception.

Fixes #28017

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• New Test case
  • Skipped for non-supported platforms
• Test case improvement

Back port request

• 202311
• 202405
• 202411
• 202505
• 202511
• 202512
• 202605

Tracking issue/work item for backport/cherry-pick request (GitHub issue or Microsoft ADO): #28017
Failure type: regression

Tested branch

• master
• 202311
• 202405
• 202411
• 202505
• 202511
• 202512
• 202605
• N/A

Test result

• master: python3 -m py_compile tests/common/helpers/telemetry_helper.py tests/ip/test_mgmt_ipv6_only.py (passed)

Approach

What is the motivation for this PR?

test_telemetry_output_ipv6_only temporarily disables gNMI user
authentication before invoking a client without credentials. The previous
supervisor reload could leave the running gNMI process using its earlier
authentication mode, causing the request to be rejected as unauthenticated.

How did you do it?

• Preserve the existing user_auth value, including an absent field.
• Restart the gNMI service after setting user_auth=none and wait for the TCP
  listener before querying it.
• Restore the original authentication configuration and restart the service in
  a finally block.
• Make the shared streaming telemetry context restore client_auth when the
  test body fails.

How did you verify/test it?

python3 -m py_compile tests/common/helpers/telemetry_helper.py tests/ip/test_mgmt_ipv6_only.py

Any platform specific information?

No.

Supported testbed topology if it's a new test case?

N/A. This updates an existing IPv6 management test.

Documentation

N/A.

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