Fix nested conditionals running skipped branches in local execution

[jackiectl2]

Tracking issue

Closes flyteorg/flyte#6426

Why are the changes needed?

In local execution, when a conditional is nested inside the active branch of another conditional (either inline in .then(...) or inside a subworkflow called from that branch), every branch of the nested conditional executes its tasks, not only the selected one. In the issue's example, top_workflow(input_value=True) runs do_test, do_nothing1 and do_nothing2, although do_test returns False and only do_nothing2 should run. The returned value is correct (the selected case's output is used), but the non-selected tasks still run, with their side effects and cost.

The cause is FlyteContext.Builder.enter_conditional_section: when already inside a condition whose branch is BRANCH_ACTIVE, it kept the parent's execution state object as-is. The nested LocalExecutedConditionalSection therefore started in BRANCH_ACTIVE, and since start_branch only ever sets the state to active, every branch ran.

What changes were proposed in this pull request?

• In local execution, every conditional section now gets its own copy of the execution state starting in BRANCH_SKIPPED, exactly as a top-level conditional already did. The nested section then activates only its selected branch, and it no longer mutates the parent's state.
• Conditionals nested in a skipped branch are unchanged: conditional() still checks the parent's state and returns a SkippedConditionalSection.
• Compilation / remote execution is unaffected (this code path only applies to local execution).

How was this patch tested?

Added two unit tests in tests/flytekit/unit/core/test_conditions.py that record which tasks actually execute:

• test_nested_condition_runs_only_selected_branch: inline nested conditional, checks the inner if / inner else / outer else paths.
• test_condition_in_subworkflow_called_from_branch: the reproduction from the issue (conditional inside a subworkflow called from a branch).

Both fail on master and pass with this change. The other unit test modules that use conditionals (test_dynamic_conditional, test_type_hints, test_workflows, test_serialization, test_gate, test_context_manager, test_promise, test_node_creation, test_references, remote/test_wrapper_classes) pass. Running all of tests/flytekit/unit/core locally gave the same set of failures with and without this change (unrelated, environment-specific: docker / optional dependencies).

Setup process

# issue reproduction; run locally with `python repro.py`
top_workflow(input_value=True)
# before: do_test, do_nothing1, do_nothing2
# after:  do_test, do_nothing2

Screenshots

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Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Related PRs

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Docs link

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