fix(security): require admin auth for MintToken in batch operations (#39)

[nanoulloa]

Summary

Closes #39.

The batch MintToken path was unauthenticated. In batch.rs,
authorize_batch_access did nothing for BatchOperation::MintToken (=> {}),
and in lib.rs the batch entry points extracted caller only from
Swap/AddLiquidity/RemoveLiquidity ops, setting caller = None for a
mint-first batch. Together these let anyone mint tokens by submitting a
batch containing a MintToken, bypassing every KYC and admin guard.

This PR makes MintToken admin-only inside batches.

Changes

• peerx-contracts/counter/batch.rs — authorize_batch_access now flags
  requires_admin on any MintToken op and, exactly once after the loop,
  calls the new require_batch_admin helper. The helper loads the configured
  admin (ADMIN_KEY), enforces admin.require_auth(), and re-checks
  require_admin, failing closed with admin_req when no admin is configured.
  Both execute_batch_atomic and execute_batch_best_effort route through
  authorize_batch_access, so both paths are covered by a single, DRY guard.
• peerx-contracts/counter/src/kyc_tests.rs — regression tests: a
  non-admin mint batch reverts on both the atomic and best-effort paths, and a
  mint batch fails closed when no admin is configured.
• scripts/check_kyc_guards.sh — extended so CI enforces the batch admin
  gate (require_batch_admin, admin.require_auth(),
  crate::admin::require_admin(env, &admin)).

Acceptance criteria

• execute_batch_atomic and execute_batch_best_effort require the admin
  when any op is MintToken (via the shared authorize_batch_access).
• Test: non-admin batch with MintToken reverts.
• check_kyc_guards.sh updated (and passing).

Verification

• scripts/check_kyc_guards.sh passes (this is the hard CI gate).
• batch.rs compiles cleanly; the added tests type-check cleanly.
• The counter crate as a whole does not yet build locally due to the
  in-progress soroban-sdk migration already tracked in CI ("user-code compile
  errors being triaged post-SDK migration"), which is unrelated to this change.
  To validate the fix behaves correctly at runtime regardless, the exact
  authorization logic was replicated in an isolated Soroban contract and
  executed against the host: an unauthorized (non-admin) mint batch reverts, an
  admin-authorized mint batch succeeds, and a mint batch with no admin
  configured fails closed — while the previous (empty-arm) logic let an
  unauthorized mint through.

[ameeribro4-sudo]

LLGTM