Feedback on public api

[sfc-gh-aalam]

1. Which Jira issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes SNOW-NNNNNNN

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   • I am adding a new automated test(s) to verify correctness of my new code
     • If this test skips Local Testing mode, I'm requesting review from @snowflakedb/local-testing
   • I am adding new logging messages
   • I am adding a new telemetry message
   • I am adding new credentials
   • I am adding a new dependency
   • If this is a new feature/behavior, I'm adding the Local Testing parity changes.
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3. Please describe how your code solves the related issue.

   Please write a short description of how your code change solves the related issue.

[sfc-gh-aalam]

I noticed we are not checking isinstance(session, Session) in all branches so this would make it more consistent

[sfc-gh-oplaton]

If we're moving away from hard-coded True, the value should come from an environment variable, or some other suitable global configuration source. We don't want to be in a place where the feature is untestable while under development/opt-in mode.

[sfc-gh-aalam]

Why would it not be testable if user has a session and they have oped-in such that session.ast_enabled returns true.

We reach this case when a valid session is not found. Now, default to assuming that ast is enabled but I suggest we default to assuming it is disabled.

[sfc-gh-oplaton]

Much of the Snowpark public API surface is session-agnostic. Hard-coding this return value to False breaks the following valid code:

  avg_a = call_function("avg", col("a"))
  with Session.builder.config(...).create() as session:
    df = session.create_dataframe([1, 2, 3, 4], schema=["a"])
    df.select(avg_a).show()