fix: Reject "multi" as the kind of an individual context

[kinyoklion]

Refs SDK-2383.

Summary

The string "multi" is reserved as the LaunchDarkly multi-context discriminator. An individual context must not use "multi" as its kind. The internal _validKind helper in packages/common/lib/src/ld_context.dart was only rejecting "kind"; building with .kind("multi", ...) was producing a valid LDContext, and a multi-context that included "multi" as one of its sub-kinds was also valid.

bool _validKind(String kind) {
-  return kind != 'kind' && _kindExp.hasMatch(kind);
+  return kind != 'kind' && kind != 'multi' && _kindExp.hasMatch(kind);
}

This propagates the kind validation Flutter SDK already implements for "kind" to also cover "multi". Both single-kind and multi-context build paths run every sub-kind through _validKind, so a single change covers both.

Test plan

• flutter test test/ld_context_test.dart — 36 tests pass (new + existing)
• flutter test (full common package) — 437 tests pass
• dart analyze --fatal-infos lib test — no issues
• dart format — no changes
• Manual: LDContextBuilder().kind('multi', 'alice').build().valid is now false; previously true
• Manual: a multi-context that includes a "multi" sub-kind builds as invalid

---

Note

Low Risk
Low risk: a small validation tightening that only changes behavior for previously-accepted invalid input (kind == "multi"). Main risk is backward incompatibility for callers incorrectly using "multi" as a kind, now producing invalid contexts.

Overview
Reserves "multi" by updating kind validation so an individual context cannot use kind == "multi", causing such contexts (and multi-contexts that include a "multi" sub-kind) to build as invalid.

Updates tests to treat "multi" as an invalid kind and adds coverage ensuring invalid contexts return empty canonicalKey and keys.

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