fix: merge credential contexts into VP @context to preserve custom claims

[wahidulazam]

Description

Fixes #893

Problem

When generating Verifiable Presentations (VPs), custom claims from embedded credentials (e.g., holderIdentifier from Tractus-X contexts) were being dropped during JSON-LD compaction. This occurred because:

1. The VP's @context array only included hardcoded base contexts:
   • https://www.w3.org/2018/credentials/v1
   • https://identity.foundation/presentation-exchange/submission/v1
2. Custom contexts from embedded credentials were not propagated to the VP
3. When the VP was compacted in PresentationApiController.createPresentation() (line 140), the JSON-LD processor dropped terms undefined in the available contexts

Solution

This PR implements context merging in LdpPresentationGenerator to extract and merge @context values from all embedded credentials with the VP's base contexts.

Key Changes:

• Added extractContextsFromCredentials() method to extract contexts from all credentials
• Added mergeContexts() method to combine base and credential contexts without duplicates
• Modified generatePresentation() to use merged contexts when building the VP
• Handles both string and array context formats gracefully
• Ensures proper ordering: W3C_CREDENTIALS_URL first, then PRESENTATION_EXCHANGE_URL, then credential contexts

Example:

Before:

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://identity.foundation/presentation-exchange/submission/v1"
  ],
  "verifiableCredential": [...]
}

After:

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://identity.foundation/presentation-exchange/submission/v1",
    "https://w3id.org/tractusx-trust/v0.8"  // ✅ Merged from credential
  ],
  "verifiableCredential": [...]
}

Testing

• Added 5 comprehensive test cases covering:
  • Custom context merging
  • Single context strings vs arrays
  • Multiple credentials with different contexts
  • Duplicate context avoidance
  • Missing contexts handling
• Added test JSON-LD context files (tractusx.v08.json, custom.v1.json)
• All 98 tests in identity-hub-core pass ✅
• Full project build successful ✅
• Checkstyle passed ✅

Files Changed

• core/identity-hub-core/src/main/java/org/eclipse/edc/identityhub/core/services/verifiablepresentation/generators/LdpPresentationGenerator.java
• core/identity-hub-core/src/test/java/org/eclipse/edc/identityhub/core/services/verifiablepresentation/generators/LdpPresentationGeneratorTest.java
• core/identity-hub-core/src/test/resources/tractusx.v08.json (new)
• core/identity-hub-core/src/test/resources/custom.v1.json (new)

Checklist

• Code follows the project's style guidelines
• Tests added for new functionality
• All tests pass
• Documentation updated where necessary
• Commit message follows conventional commits format
• Issue reference included in commit message

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Branch: fix/893-merge-credential-contexts-vp
Commit: 39833797
Ready for review 🚀