fix: resolve recommendations #1-#4 from model-bindings quality report

[mwiebe]

What was the problem/requirement? (What/Why)

Agent-generated report found issues to fix up.

What was the solution? (How)

Addresses the first four recommendations in
reports/model-bindings-quality-evaluation-report.md. Each item is struck through in the report with a **Resolved** note.

#1 — Document empty-steps decode error class divergence (docs)

Empty-steps decode failures surface under ModelValidationError rather than v0's DecodeValidationError. The v1 binding splits decode-stage (schema-level) failures from model-stage (structural-invariant) failures, with the empty-steps check living in the model validator — this is a deliberate v0 divergence, not a bug.

• spec's 'Exceptions' section updated: corrected empty-steps example, divergence note explaining the split and recommended catch-tuple
• strengthened the existing regression test in test_parse.py to assert the full message body per the AGENTS.md test quality standard

#2 — Relocate resolved TemplateSpecificationVersion gap test (test)

The Python str-Enum shim for TemplateSpecificationVersion was already removed in a prior change; the Rust pyclass at openjd._openjd_rs.TemplateSpecificationVersion is the single canonical class, re-exported identity-preserving from openjd.model._v1. The regression test had become passing (no xfail marker) but was still parked in test_known_gaps.py.

• test moved to test_version_enums.py::TestTemplateSpecificationVersion ::test_template_specification_version_returned_from_decode with strengthened identity / value / string-equality assertions
• spec's 'Pickle Support' section: stale 'str-Enum shim' paragraph corrected — pickle goes directly through the module-level _reconstruct_enum helper; pickle table now lists both spec-revision enums explicitly

#3 — Emit merge defaults in native Python types (fix)

The upstream MergedParameterDefinition::default is Option<String> — every variant is stringified through default_value(). The binding contract per the spec is that callers receive the default in its native Python type (int for INT, float for FLOAT, list[T] for LIST[T], etc.).

• new default_to_native helper in create_job_fns.rs dispatches on JobParameterType and parses the stringified default back via str::parse / serde_json::from_str; parsing failures fall back to the raw string as a defensive guard
• pre-existing parametrized expectations in test_merge_job_parameters.py::TestMergeTemplates_v2023_09 updated to native form ('default': 8 vs 'default': '8')
• two int/float xfail tests moved out of test_known_gaps.py and expanded into a full TestMergeDefaultNativeTypes class covering all 10 type variants with type-identity assertions

#4 — Propagate parameter description through merge (feat)

The upstream MergedParameterDefinition struct does not surface a description field — only name / param_type / default / object_type / data_flow / source / merged constraints. v0 carried it on the typed pyclass merged result, so v1 was losing per-parameter human-readable text that downstream tooling (deadline-cli's parameter-prompt UI) relies on for parameter labels.

• py_merge_job_parameter_definitions now walks env templates in order then the job template, building a name→description HashMap. Later descriptions overwrite earlier ones, matching how the upstream merge tracks default
• the merged dict carries the description key only when at least one contributing template provided one (consistent with how default / objectType / dataFlow are conditionally emitted)
• spec's 'Return shape' key list now lists description with its last-wins ordering semantics
• xfail test moved out of test_known_gaps.py and expanded into a 5-case TestMergeDescriptionPropagation class

Verification

• python -m pytest test/openjd/model_v0 test/openjd/model_v1 → 3269 passed (+5 from the new tests), 0 xfails
• python -m pytest test/ → 5131 passed, 24 skipped, 3 unrelated expr xfails
• cargo clippy --manifest-path rust-bindings/Cargo.toml --all-targets -- -D warnings → clean
• No public binding signatures changed; _openjd_rs.pyi unchanged

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