feat(editor): Add node-context chat telemetry (no-changelog)

[jgalmeida]

Summary

Adds telemetry for the "add nodes to chat" (node context) feature, per https://linear.app/n8n/issue/ADO-5793. Two events in the INSTANCE_AI domain of the @n8n/telemetry registry.

Stacked on the feature branch (base is claude/ado-5773-front-end-bc4413, PR #36225), so the diff here is only the telemetry commit.

Events

Event	Properties	Fires
User added nodes to chat	source, node_count	The shared addSelectedNodesToChat chokepoint, after the attachment is built — so node_count is the count actually attached (unresolved ids dropped, capped at the per-set max), not the raw selection. source is threaded from each Canvas trigger.
User sent chat message with nodes	node_count	Both send paths (empty view + thread view), only when the message carries node context. node_count totals nodes across every attached set.

source values: node_toolbar, selection_toolbar, context_menu, group_title_bar, keyboard.

Kept intentionally minimal (source + count) — no thread_id/instance_id (PostHog already carries session identity), no set-count.

Testing

• FE unit tests: 6 new (4 for useAddNodesToChat source/count, 2 for the countAttachedNodes helper); mock useTelemetry().track and assert the registry entry + payload.
• @n8n/telemetry registry checks pass (descriptions non-blank, JSON-schema-representable).
• pnpm typecheck (editor-ui) clean — confirms every track() payload matches its zod schema.
• eslint clean on all touched files.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...src/features/ai/instanceAi/InstanceAiEmptyView.vue	50.00%	1 Missing and 1 partial ⚠️
...rc/features/ai/instanceAi/InstanceAiThreadView.vue	50.00%	1 Missing ⚠️
...rc/features/workflows/canvas/components/Canvas.vue	0.00%	1 Missing ⚠️

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