[Container] az container exec: fix exception when stdin is not a tty

[micromaomao]

Reproduction steps:

az container exec \
  --container-name "container" \
  -g "$RESOURCE_GROUP" \
  -n "$ACI_NAME" \
  --exec-command "echo Hello" > ./stdout < /dev/null 2>./stderr

Expected behavior:

./stdout should contain "Hello"

Actual behavior:

Command crashes without running the command. Error:

 ERROR: The command failed with an unexpected error. Here is the traceback:
 ERROR: (25, 'Inappropriate ioctl for device')
 Traceback (most recent call last):
 File "/opt/az/lib/python3.12/site-packages/knack/cli.py", line 233, in invoke
 cmd_result = self.invocation.execute(args)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/opt/az/lib/python3.12/site-packages/azure/cli/core/commands/__init__.py", line 666, in execute
 raise ex
 File "/opt/az/lib/python3.12/site-packages/azure/cli/core/commands/__init__.py", line 733, in _run_jobs_serially
 results.append(self._run_job(expanded_arg, cmd_copy))
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/opt/az/lib/python3.12/site-packages/azure/cli/core/commands/__init__.py", line 703, in _run_job
 result = cmd_copy(params)
           ^^^^^^^^^^^^^^^^
 File "/opt/az/lib/python3.12/site-packages/azure/cli/core/commands/__init__.py", line 336, in __call__
 return self.handler(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/opt/az/lib/python3.12/site-packages/azure/cli/core/commands/command_operation.py", line 121, in handler
 return op(**command_args)
           ^^^^^^^^^^^^^^^^^^
 File "/opt/az/lib/python3.12/site-packages/azure/cli/command_modules/container/custom.py", line 970, in container_exec
 _start_exec_pipe_linux(execContainerResponse.web_socket_uri, execContainerResponse.password)
 File "/opt/az/lib/python3.12/site-packages/azure/cli/command_modules/container/custom.py", line 1006, in _start_exec_pipe_linux
 old_tty = termios.tcgetattr(stdin_fd)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 termios.error: (25, 'Inappropriate ioctl for device')
 To check existing issues, please visit: https://github.com/Azure/azure-cli/issues

This makes the "exec" command unusable in e.g. CI environments.

Tested that all of the following works:

• Running az container exec normally, starting bash, starting vi and it takes the correct terminal size.
• Running az container exec in our CI now works
• The provided reproduction command now no longer crashes.

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[yonzhan]

Thank you for your contribution! We will review the pull request and get back to you soon.