[wolf4ood]

What this PR changes/adds

The onboarding agent no longer sends a holderPid when it requests credentials from IdentityHub. It now reads the holderPid that IdentityHub assigns from the Location header of the response.

• CredentialRequest no longer has a HolderPID field.
• New identityhub.HolderPIDFromLocation(location) returns the last path segment of the Location header (.../credentials/request/{holderPid}). It returns an error if the segment is missing.
• The onboarding activity stores the holderPid returned by IdentityHub. It then polls the request state with it, as before.

Why it does that

Since eclipse-edc/IdentityHub#1219 , IdentityHub assigns the holderPid itself. It rejects any credential request that already carries one:

400 [{"message":"Property 'holderPid' must not be set, it is assigned by IdentityHub","type":"InvalidRequest"}]
This breaks participant onboarding with EDC 1.0.0-RC5.

Further notes

• Requires an IdentityHub version that includes #1219, such as 1.0.0-RC5. Older IdentityHub versions ignore a request without holderPid, so the agent can't track the request against them.
• Tests: the CredentialRequest serialization tests are updated, with a new check that holderPid is never serialized. There are new tests for HolderPIDFromLocation. The onboarding deploy test now checks that the stored holderPid comes from the location.