Surface block-level contextual objects in TAE snippets

[graygilmore]

Summary

In a theme app extension, snippets can only be rendered from blocks/*.liquid files, so the ambient context always includes the block-level objects (section, block, recommendations, app). Previously, the completion and diagnostic systems only provided app for snippets regardless of project mode — no autocomplete or hover for section, block, or recommendations in TAE snippet files.

Commit 1: Thread project mode through TypeSystem and UndefinedObject check

Adds an optional getModeForURI callback to TypeSystem, CompletionsProvider, and HoverProvider, and passes context.mode into the UndefinedObject check. No behavior changes — pure plumbing so that downstream code can branch on project mode.

Commit 2: Surface block-level contextual objects in TAE snippets

When mode is app, snippets now get the same contextual objects as blocks: section, block, recommendations, app. This applies to both completions and the UndefinedObject diagnostic.

Closes #346

🎩 Tophatting

1. Open a theme app extension project in VS Code with the Shopify Liquid extension
2. Create or open a snippets/*.liquid file
3. Type {{ section — you should now see section in the autocomplete suggestions
4. Verify block, recommendations, and app also appear
5. Verify there are no UndefinedObject warnings for these objects
6. Open a regular theme project (not a TAE) and confirm snippets still only get app — section, block, and recommendations should NOT appear

[graygilmore]

Instead of threading mode all the way through we could do something really lazy here but the cost doesn't seem too bad and it sets up a pattern that we might want to use in other areas.