Parse only the query component in uriSchemePresentment()

[kthrtty]

Fixes #2032

parseUrlEncodedParameters() splits on & and then on the first =, with no notion of a scheme or a ?, so handing it a whole URI folds the scheme into the first key. openid4vp://?request_uri=... yielded the single key openid4vp://?request_uri and the lookup threw No request_uri.

The call only worked when some other parameter preceded request_uri. Even then client_id itself was unreadable for the same reason, its key being openid4vp://?client_id; it simply went unused, which is why this stayed hidden.

OpenID4VP 1.0 puts client_id inside the signed request object and section 5.10 lets the Authorization Request reference it by request_uri, so a verifier that signs its request has no obligation to repeat client_id in the outer query. request_uri on its own is a request a conforming verifier may send.

Approach

Takes the substring after the first ? rather than going through Url, whose handling of a custom scheme with an empty authority is less obvious in this context. A URI with no ? yields no parameters, which reaches the same No request_uri as before.

The helper is internal so the parsing can be tested on its own; uriSchemePresentment() itself needs an HTTP client and a PresentmentSource.

Verification

UriSchemeQueryParsingTest covers five cases: request_uri alone, request_uri after client_id, the resulting key names, request_uri_method, and a URI with no query.

Three of them fail against the previous implementation, so they hold the regression rather than merely passing:

FAIL  requestUriAsOnlyParameter
FAIL  requestUriAfterClientId
FAIL  schemeIsNotFoldedIntoTheFirstKey
ok    requestUriMethodIsRead
ok    noQueryYieldsNoParameters

Ran :multipaz:jvmTest and detektMetadataCommonMain.

Found while wiring the iOS wallet's custom URL schemes (openwallet-foundation/multipaz-wallet#35).

Not included

No CHANGELOG entry: the 0.102.0 section lists feature-sized items and this is a small parsing fix. Happy to add one if you consider it notable.