feat(hts): emit multi-issue OperationOutcomes from $validate-code

[smunini]

The HL7 tx-ecosystem IG fixtures expect $validate-code failure cases to surface one OperationOutcome.issue per concern (abstract + not-in-vs, unknown-system + not-in-vs, inactive + valid-but-not-active + not-in-vs, display-mismatch, etc.) rather than collapsing every signal into a single error issue. The previous implementation always emitted a single issue with code=code-invalid + tx-issue-type=not-in-vs, which made ~40 fixtures fail with "Expected:N Actual:1" issue counts.

Changes:

• types.rs: add ValidationIssue (severity, fhir_code, tx_code, text, location, message_id) and ValidateCodeResponse.issues field — backends push one issue per detected concern; the operations layer renders them as OperationOutcome.issue[] entries and joins their texts (alphabetically, "; "-separated) into the top-level message parameter.
• backends/sqlite/value_set.rs::finish_validate_code_response: emit separate issues for code-not-in-vs, abstract (business-rule code-rule
  • code-invalid not-in-vs), inactive (warning), display-mismatch (error invalid-display), and the IG-specific "valid but not active" triple when a code is in the underlying CodeSystem but excluded from the VS by activeOnly / compose.inactive=false.
• backends/sqlite/code_system.rs: emit not-found/not-found for unknown CodeSystem, code-invalid/invalid-code for unknown code, and invalid/invalid-display for display mismatch.
• operations/validate_code.rs::build_validate_response: walk resp.issues; synthesise an UNKNOWN_CODESYSTEM not-found issue when the operations layer sees an unknown system (surfaces the IG-required second issue alongside the backend's not-in-vs); fall back to the legacy single-issue path only when no structured issues are present.
• backends/postgres/*: add empty issues field to satisfy the new struct shape (postgres backend keeps single-message behaviour for now).

Adds two unit tests (vs_validate_unknown_system_emits_two_issues, vs_validate_no_system_on_coding_emits_invalid_data_issue) covering the two simplest multi-issue scenarios. All 622 helios-hts tests pass.

[skip ci]