feat(hts): IG Publisher $validate-code compatibility

[sandhums]

Pull request: HTS IG Publisher $validate-code compatibility

Summary

HL7 FHIR IG Publisher (2.2.x) validates Implementation Guides against a terminology
server via -tx. HTS previously reported dozens of false-positive validation errors
even when terminology was loaded and manual $validate-code probes succeeded.

This PR aligns HTS with Publisher request shapes and tx.fhir.org behaviour for:

• Parameter name aliases (valueCoding, valueCodeableConcept, and system for CodeSystem URL)
• SNOMED version=current resolution
• LOINC import version labeling and stub ordering
• BCP-13 / inline mimetypes ValueSet validation
• Inline ValueSet + codeableConcept (Composition bindings)
• Handler cache policy (only cache result=true)
• UCUM composed units (mg, etc.) via structural validation

Validated against Atrius IG: Publisher errors 64 → 0 after rebuild, TX cache clear,
and HTS kept up through validation.

Full design notes: ig-publisher-compatibility.md

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CodeSystem system parameter (spec vs Publisher)

FHIR spec: CodeSystem $validate-code bare-code form uses in-parameter url for the
CodeSystem canonical.

IG Publisher: Often sends system + code (e.g. Library dataRequirement / LOINC
validation), not url + code.

HTS behaviour: On CodeSystem Path 1 (bare code), accept system as an alias for
url. Prefer url when both are present. Return 400 only when neither is supplied.

Test change: Replaced system_param_rejected_with_400 (asserted spec-only rejection)
with system_param_alias_validates_code (asserts 200 and result=true for Publisher
shape). This documents the intentional compatibility choice for upstream review.

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Test plan

• cargo fmt --check
• cargo clippy -p helios-hts -- -D warnings
• cargo test -p helios-hts
• Manual probes (HTS on :9091):
  • CodeSystem: system + code (LOINC 85354-9)
  • CodeSystem: UCUM mg on http://unitsofmeasure.org
  • ValueSet: inline mimetypes + code=text/cql (no url)
  • ValueSet: inline VS + valueCodeableConcept (no url)
• Optional: IG Publisher build with -tx http://127.0.0.1:9091 and cleared
  input-cache/txcache

Key automated tests:

Area	Test
system alias	system_param_alias_validates_code
Parameter aliases	extract_coding_accepts_value_coding_alias, etc.
Inline mimetypes	vs_inline_mimetypes_publisher_shape_validates_text_cql
Inline CC	vs_inline_codeable_concept_publisher_shape_validates
UCUM composed	cs_ucum_composed_mg_validates_when_not_in_essence_table, ucum_validate::tests::*
SNOMED current	validate_code_version_current_resolves_loaded_edition

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Files touched (high level)

See ig-publisher-compatibility.md § Files touched.

Exclude from PR (formatting-only, no behaviour change): ecl/*, sqlite/mod.rs if
present as unrelated diffs.

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Breaking changes

None for production API consumers. Test expectation change only: integration test
system_param_rejected_with_400 removed in favour of system_param_alias_validates_code
because Publisher compatibility requires accepting system on the bare-code path.