chore: assert amulet.admin is present before building commands that need it (#2326)

* Assert amulet.admin is present before building commands that need it

fetchAmulet() returns AssetBody with admin: PartyId typed as always
present, but parseAssets does no runtime validation of the registry
response it comes from. Three call sites (preapproval create command,
traffic buyMemberTraffic, namespace tap) pass amulet.admin straight
into ledger commands unchecked -- if a registry response is missing
it, this silently builds a command (e.g. transfer preapproval) with
an undefined expectedDso instead of failing loudly.

Add one guard in fetchAmulet() that protects all three call sites.

Fixes #1797

Signed-off-by: ayushsingh82 <ayushsinghmi711@gmail.com>

* Move admin (DSO) party validation into parseAssets

Validating the registry response's admin field in fetchAmulet() only
covered the amulet call site. parseAssets is the shared function that
converts raw registry data into AssetBody for all callers (amulet,
token transfer, token utxos), so the guard belongs there instead.

Adds a unit test covering the missing-admin case.

Signed-off-by: ayushsingh82 <ayushsinghmi711@gmail.com>

* Add explicit return type to parseAssets

Address review feedback: annotate the return type as AssetBody[]
instead of relying on inference.

Signed-off-by: ayushsingh82 <ayushsinghmi711@gmail.com>

---------

Signed-off-by: ayushsingh82 <ayushsinghmi711@gmail.com>
Co-authored-by: rukmini-basu-da <126689545+rukmini-basu-da@users.noreply.github.com>