test: use walletkit-testkit for core proof-generation integration tests

[kilianglas]

Summary

Migrates walletkit-core's proof-generation integration tests onto the walletkit-testkit helpers, removing a second hand-rolled copy of the full staging proof flow and its duplicated hardcoded fixtures (RP id, issuer schema, keys, verifier, RPC), which now live centrally in TestEnv.

Changes

• Add walletkit-testkit as a dev-dependency of walletkit-core (path-only, publish = false, so it is stripped from the published manifest). Add tempfile as a dev-dependency.
• Rewrite crates/walletkit-core/tests/proof_generation_integration.rs:
  • e2e_authenticator_generate_proof collapses to a single generate_and_verify_test_proof(CredentialType::Local, .., ProofType::Uniqueness) call (~200 lines → a few).
  • e2e_session_proof is rebuilt on the lower-level helpers (init_and_register_account, issue_local_credential, build_test_request), keeping the parts unique to this crate: the follow-up ProofType::Session proof and the WIP-103 ownership-proof Merkle-root cross-check.
  • Both tests keep running by default (not #[ignore]d), exactly as before the migration — this PR does not change which tests run in CI.
• TestEnv::default_staging() now honors the WORLDCHAIN_RPC_URL env var (for both the verifier RPC and the World ID config), preserving the migrated tests' previous behavior — CI passes this from a secret.
• Remove build_base_credential from tests/common.rs — it was a verbatim duplicate of walletkit_testkit::issuer::build_base_credential and is no longer referenced.

Test plan

• cargo clippy --workspace --all-targets -- -D warnings — clean.
• cargo fmt --check — clean.
• cargo package -p walletkit-core — packaged manifest contains no walletkit-testkit reference (dev-dep stripped), confirming publish safety.
• The migrated e2e tests run against staging in the regular test suite:
  cargo test -p walletkit-core --test proof_generation_integration --features embed-zkeys