latch preview does not send flow metadata (Spoiler, Section, Params not rendered)

[nh13]

Problem

latch preview ignores the flow array in LatchMetadata. Workflows using flow= (with Section, Spoiler, Params, etc.) render with the old default layout and a "Hidden Parameters" fallback instead of their defined UI structure. This forces a full latch register cycle just to verify layout changes.

Root cause

preview.py sends only per-parameter data via wf.interface.to_flyte_idl().inputs. The workflow-level __metadata__ dict — which contains the flow array — is serialized into the workflow's docstring by _inject_metadata() but is never extracted or forwarded by the preview service.

The upsertWorkflowPreview mutation only accepts argAccountId and argInputs, with no field for workflow-level metadata.

Proposed fix

SDK side (src/latch_cli/services/preview.py): Extract the __metadata__ block from the workflow docstring and pass it to the mutation.

import yaml

# After obtaining `wf`, extract workflow-level metadata from docstring
metadata = {}
docstring = wf.python_interface.docstring or ""
if "__metadata__:" in docstring:
    yaml_start = docstring.index("__metadata__:")
    try:
        parsed = yaml.safe_load(docstring[yaml_start:])
        if isinstance(parsed, dict):
            metadata = parsed.get("__metadata__", {})
    except yaml.YAMLError:
        pass

# Include in mutation
execute(
    gql.gql("""
        mutation UpdatePreview($accountId: BigInt!, $inputs: String!, $metadata: JSON) {
            upsertWorkflowPreview(
                input: { argAccountId: $accountId, argInputs: $inputs, argMetadata: $metadata }
            ) {
                clientMutationId
            }
        }
    """),
    {
        "accountId": current_workspace(),
        "inputs": MessageToJson(wf.interface.to_flyte_idl().inputs),
        "metadata": metadata if metadata else None,
    },
)

Backend side: Add an optional argMetadata parameter (JSON) to the upsertWorkflowPreview mutation and persist it so the preview page can use flow for rendering, the same way registered workflows do.

Fallback: When metadata is None or flow is absent, behavior is unchanged — the frontend falls back to per-parameter _tmp.hidden and section_title layout.

Reproduction

from latch.resources.workflow import workflow
from latch.types.metadata import LatchMetadata, LatchParameter, Params, Section, Spoiler

metadata = LatchMetadata(
    display_name="Test Spoiler Preview",
    parameters={
        "name": LatchParameter(display_name="Name"),
        "advanced_opt": LatchParameter(display_name="Advanced Option"),
    },
    flow=[
        Section("Inputs", Params("name")),
        Spoiler("Advanced", Params("advanced_opt")),
    ],
)

@workflow(metadata)
def test_wf(name: str = "hello", advanced_opt: int = 42) -> str:
    ...

Run latch preview . — sections and spoiler are not rendered.