Finish TryGetValue cleanup in FromEventJson implementations

[Copilot]

Follow-up to #365, which converted the 21 ContainsKey + indexer lookups it had already touched (b304ed6). The remaining 63 sites across 23 hand-maintained event-args/component files under src/components/Blazor still did a double dictionary lookup; this converts them to a single TryGetValue.

Changes

• Single-lookup conversion — every if (args != null && args.ContainsKey("x")) { … args["x"] … } in a FromEventJson body now uses TryGetValue, with the object? result passed straight to the existing ReturnTo* / ConvertReturnValue / StringToEnum helper.
• Local naming — <key>Obj, matching the convention established in b304ed6; no collisions with existing pattern variables such as detail.
• Shape normalization — TileChangeStateEventArgs used the args?.ContainsKey("detail") == true && … args["detail"] variant; it now matches the same args != null && args.TryGetValue(...) form as its siblings.
• Scope — hand-maintained files only. The other ContainsKey uses in src/componentsBase/BaseRendererControl.cs are internal state lookups, not event payload reads, and are untouched.

// before
if (args != null && args.ContainsKey("attachments"))
{ this.Attachments = ReturnToObjectArray<IgbChatMessageAttachment>(args["attachments"]); }

// after
if (args != null && args.TryGetValue("attachments", out var attachmentsObj))
{ this.Attachments = ReturnToObjectArray<IgbChatMessageAttachment>(attachmentsObj); }

Behavior is unchanged — TryGetValue returning false leaves out at null, which the guarded body never reads, so each site keeps the same "only assign when the key is present" semantics.

No new tests: the refactor is behavior-preserving and these decode paths are already exercised by the existing event tests (e.g. ComboTests drives ComboChangeEventArgsDetail.FromEventJson).