fix: tolerate empty deepLinkData object in InstallResponse

[onamfc]

Ports the iOS SDK fix (mobile-sdk-ios#4) to Flutter. The bug was reported against iOS, but Flutter has the same defect.

Problem

The install endpoint returns deepLinkData: {} rather than null for organic (unattributed) installs:

{
  "installId": "<some_id>",
  "attributed": false,
  "confidenceScore": 0,
  "matchedFactors": [],
  "deepLinkData": {}
}

DeepLinkData.shortCode is required, so the generated DeepLinkData.fromJson threw a TypeError on json['shortCode'] as String. That happened inside InstallResponse.fromJson, so the whole response failed to parse and initialize() threw for every organic install — not just the deep link part of it.

Fix

deepLinkData now parses through @JsonKey(fromJson: _deepLinkDataFromJson): anything that isn't a map, or that DeepLinkData.fromJson rejects, becomes null — so {} and a short-code-less object mean "no deep link", the same as null and absent. The failure is logged when debug logging is on rather than rethrown, since a deep link the SDK can't parse is never worth aborting attribution over.

Deliberately unchanged:

• DeepLinkData.shortCode stays required. A deep link without a short code isn't routable, and loosening it would be a breaking change for anything reading it.
• Top-level fields stay strict. installId, attributed, confidenceScore, and matchedFactors are always returned, and installId is required for the SDK to work at all.
• toJson is untouched — the generated serializer still emits deepLinkData?.toJson().

install_response.g.dart is regenerated with build_runner; the only change is the one call site.

Fixing this SDK-side rather than only backend-side is what actually unblocks integrators: self-hosted deployments and already-deployed backends will keep sending {}, and the SDK has to survive that.

Tests

install_response_test.dart gains coverage for {}, null, and short-code-less deepLinkData, the attributed path, and a missing-required-field failure. Full suite: 106 tests passing, flutter analyze clean.