Fix response contracts in Discovery OpenAPI files (3.2 Port)

[aaronzi]

This pull request updates the AAS Discovery response contract for POST /lookup/shells/{aasIdentifier} to align with the operation’s effective create-or-replace behavior. Specifically, 404 Not Found and 409 Conflict were removed from the documented responses for this operation, so the contract now reflects successful create/update via 201, invalid requests via 400, and generic errors via default. The changelog for 3.1.3 was also extended to document both fixes.

API Specification Fixes:

• Updated V3.1_SSP-001.yaml for PostAllAssetLinksById:
  • Removed 404 Not Found from POST /lookup/shells/{aasIdentifier}
  • Removed 409 Conflict from POST /lookup/shells/{aasIdentifier}
• Mirrored the same response-contract changes in V3.1.yaml to keep the aggregated OpenAPI specification consistent.

Documentation Updates:

• Updated changelog.adoc under Changes w.r.t. V3.1.2 to V3.1.3 (Minor Changes):
  • Added entry for issue 595 (remove 404 for POST /lookup/shells/{aasIdentifier})
  • Added entry for issue 596 (remove 409 for POST /lookup/shells/{aasIdentifier})

Related Issues

See #595 and #596

Cherry picked bf09788 from #616

IMPORTANT!

Merge #602 first