feat(metadata): add AppMetadata value class (pluggable-encryption provider metadata)

[gkc]

What

Adds an AppMetadata value class to at_client and exposes it as
Metadata.appMetadata, mirroring the canonical Dart at_commons
AppMetadata (at_key.dart).

• AppMetadata value class — an SDK-owned providerId routing field plus
  an opaque additional map the SDK preserves but does not interpret. Provides:
  • toJson() → the flat object { "providerId": …, …additional } (additional
    entries serialised flat, not nested — matching canonical).
  • fromJson(Map) → routes every key other than providerId into additional;
    throws on a missing/blank providerId.
  • encode() → base64(JSON(toJson())), the wire form.
  • decode(Object) → accepts a base64(JSON) String (wire), a flat Map
    (metadata-map form), or an already-parsed AppMetadata; null/"null" → null.
• Metadata.appMetadata typed as AppMetadata. toString() appends
  :appMetadata:<base64(JSON)> last, after immutable, matching the
  canonical fragment order. Static Metadata.encodeAppMetadata /
  Metadata.decodeAppMetadata delegate to the value class for API parity with
  the Dart client, and Jackson (@JsonValue / @JsonCreator) round-trips the
  flat-object form used in metadata maps.

Why

appMetadata is the provider-owned crypto metadata the SDK stamps on encrypted
records. Its providerId selects the crypto provider able to decrypt the value
— the routing key of the pluggable encrypt/decrypt provider model. The Java
client is a fast follower to implement that model as part of adopting the
post-quantum-safety enhancements currently in flight on the Dart client SDK, and
the provider model depends on AppMetadata; this change lands the value class so
that work has it to build on.

It also unblocks a Java atServer implementation, which needs AppMetadata on
Metadata to parse, store, and re-emit appMetadata through llookup:all /
sync / notify — the receiver relies on providerId to pick the right decryptor
after sync or cached delivery. Without it the field is dropped and pluggable-
encryption round-trips fail.

Canonical parity

Aspect	Canonical (at_commons AppMetadata)	This change (at_client)
Model	providerId + additional value object	providerId + additional value class
toJson	flat {providerId, …additional}	flat {providerId, …additional}
fromJson	non-providerId keys → additional; validates	non-providerId keys → additional; validates
Wire encoding	base64(JSON) via encodeAppMetadata	base64(JSON) via encode() / encodeAppMetadata
Decode	String(base64) / Map / AppMetadata via decodeAppMetadata	same, via decode() / decodeAppMetadata
Fragment position	last, after immutable	last, after immutable

Tests

MetadataTest +8 (72 total): toJson/fromJson flattening + validation,
encode/decode round-trip (incl. the static Metadata helpers), decode
of Map/AppMetadata/null, the base64 toString() fragment, and Jackson
deserialisation of a nested appMetadata object.