Format2 schema accepts documents missing required `class: GalaxyWorkflow` field

[jmchilton]

Problem

Format2MissingClass is listed as a schema-decode-enforced rule, but the current pydantic models accept Format2 workflows missing the class: GalaxyWorkflow field under both lax and strict validation.

Repro

from gxformat2.schema.gxformat2 import GalaxyWorkflow as Lax
from gxformat2.schema.gxformat2_strict import GalaxyWorkflow as Strict

wf = {
    "inputs": {"x": {"type": "File"}},
    "outputs": {"o": {"outputSource": "cat/out_file1"}},
    "steps": {"cat": {"tool_id": "cat1", "in": {"input1": "x"}}},
}
Lax.model_validate(wf)     # passes
Strict.model_validate(wf)  # passes — should fail

Root cause

gxformat2/schema/gxformat2_strict.py:498 (and gxformat2.py:482):

class_: Literal["GalaxyWorkflow"] = Field(default="GalaxyWorkflow", alias="class")

The default= makes the field optional. The source schema schema/v19_09/workflow.yml:852 declares class as a required enum — no default/null intended.

Impact

• Schema-rule catalog cannot anchor Format2MissingClass with a negative fixture — the rule was dropped from gxformat2/schema_rules.yml in the initial landing because negatives pass validation.
• Downstream consumers relying on decode to reject class-less Format2 documents currently get silent acceptance.

Proposed fix

Remove the default= from the generated class_ field for GalaxyWorkflow in both lax and strict models. This likely means adjusting the schema-salad-plus-pydantic codegen (or the source schema annotation) so closed-enum single-symbol fields emit as required Literal[...] without a default.

Same concern may apply to SubworkflowMissingSteps — nested subworkflow validates with empty/missing steps. Worth auditing in the same fix.