[flytekit]: fix pod_template serialization for dynamic task node overrides

[Vervious]

Why are the changes needed?

When a dynamic task uses with_overrides(pod_template=...), the pod spec in the TaskNodeOverrides was only serialized when should_fast_serialize() returned True. During dynamic task compilation at runtime, should_fast_serialize() is typically False, so override_pod_spec remained {} (empty dict). However, the TaskNodeOverrides still constructed a PodTemplate(pod_spec={}, ...) because entity._pod_template is checked separately downstream. This resulted in the propeller receiving a node with an empty pod spec override, causing the child task to stay in UNKNOWN status.

This was observed in execution byc-812vfva9fvq7 where a post-training dynamic task's child showed UNKNOWN status indefinitely.

The fork had diverged from upstream flytekit here — upstream always calls _serialize_pod_spec when a pod_template exists, and only gates set_command_fn behind should_fast_serialize().

What changes were proposed in this pull request?

Moved _serialize_pod_spec outside the should_fast_serialize() gate so it's called whenever entity._pod_template is not None, matching upstream behavior. The set_command_fn call (which rewrites the entrypoint for fast registration) remains gated behind should_fast_serialize() with the existing ContainerTask guard.

Before:

if entity._pod_template is not None and settings.should_fast_serialize():
    if not isinstance(entity.flyte_entity, ContainerTask):
        entity.flyte_entity.set_command_fn(...)
    override_pod_spec = _serialize_pod_spec(...)

After:

if entity._pod_template is not None:
    if settings.should_fast_serialize() and not isinstance(entity.flyte_entity, ContainerTask):
        entity.flyte_entity.set_command_fn(...)
    override_pod_spec = _serialize_pod_spec(...)

How was this patch tested?

• Existing translator unit tests pass (7/7).
• Manual code review confirming alignment with upstream flytekit.

Human review checklist

• Verify that _serialize_pod_spec + _get_container produce valid output when set_command_fn has NOT been called (non-fast-serialize path). This is the main behavioral change.
• Confirm that the original fork divergence (adding the ContainerTask isinstance check) is preserved in the new structure.
• Consider whether a regression test for pod_template serialization without fast_serialize is warranted.

Check all the applicable boxes

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

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Link to Devin run: https://app.devin.ai/sessions/2d5b0a201d6d472db332fd9f20485019
Requested by: @Vervious

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