Implement DynamicsExporter: Payload Serialization

Use `dart:convert`'s `jsonEncode` (via `json.encode`) for all serialization — never build JSON via `StringBuffer` concatenation. Define a `toJson()` method on `DynamicsMappedRecord` that returns `Map` with the exact portal field names. The top-level batch envelope should be built in `DynamicsExporter._buildBatchEnvelope()` as a private method, keeping `export()` readable. Unlike `XledgerExporter` which produces both CSV and JSON, `DynamicsExporter` produces JSON only — do not add CSV output unless HLF explicitly requests it.

The `exportedAt` timestamp should be UTC (`DateTime.now().toUtc().toIso8601String()`). The `CsvJsonFileGenerator` component can still be used for the JSON serialization path if it supports JSON array/object output — use it for consistency, but do not force a CSV code path. Confirm with HLF whether the Dynamics portal import accepts a raw JSON file upload or requires a multipart form POST — the serialization layer (this task) is the same either way, but the upload mechanism will differ and should be noted in the API integration task.

Unit tests using `flutter_test`. Required scenarios: (1) 3-record batch produces valid JSON parseable by `jsonDecode`; (2) top-level structure has `batchReference`, `exportedAt`, `records` keys; (3) amount `150.00` serialized as JSON number `150.00`, not `"150.00"` string; (4) date serialized as ISO 8601 UTC string; (5) `batchReference` in payload matches `exportRunId` in wrapper; (6) empty mapped records returns `ValidationError`; (7) `exportedClaimIds` in wrapper matches input; (8) JSON with a string field containing special characters (`&`, `<`, `>`, `"`) is correctly escaped. Golden file snapshot test: generate JSON from fixed 2-record input and assert exact output matches stored fixture.