Add explicit Task coding validation for CodeSystem and ValueSet with separate ERROR issues

[khalilmalla95]

Description

Current Task terminology validation checks coding.system + coding.code via cache lookup and emits a generic error when unknown.
However, we need stricter and more explicit validation for Task.input.type.coding entries, including separate issue types and explicit ValueSet validation.

For Task inputs like:

<input>
  <type>
    <coding>
      <system value="http://example.org/fhir/CodeSystem/tutorial"/>
      <code value="tutorial-input"/>
    </coding>
  </type>
  <valueString value="Hello from the DSF tutorial – this is my tutorial-input for Exercise 2."/>
</input>

we want the following behavior.

Required validations

1. Validate coding.system against known CodeSystem resources

   • If coding.system URI is not found in available CodeSystem resources, emit ERROR.
   • Create a dedicated issue for this case.

2. Validate coding.system against relevant ValueSet resources

   • Check whether the system is referenced by the expected ValueSet context (profile/binding-driven or configured rule scope).
   • If system is not part of the expected ValueSet context, emit ERROR.
   • Create a dedicated issue for this case.

3. Validate coding.code against CodeSystem under the relevant URI

   • Resolve code validity specifically under the provided coding.system URI (important when multiple CodeSystems exist).
   • If code is missing in that specific CodeSystem, emit ERROR.
   • Create a dedicated issue for this case.

Why

• Current behavior is too coarse-grained for diagnostics (unknown code only).
• We need actionable feedback:
  • system unknown
  • system not allowed by ValueSet context
  • code invalid for that exact system URI
• This is especially important when multiple CodeSystem resources exist and code values overlap across systems.