feat(tools): add Google Discovery API URLs to WASM tool descriptions (#585)

Add Google Discovery Service URLs to all 6 Google WASM tool
descriptions so the LLM can fetch full API documentation on demand
using its built-in HTTP tool. Discovery API is public and requires
no authentication.

URLs added:
- Gmail: googleapis.com/discovery/v1/apis/gmail/v1/rest
- Calendar: calendar-json.googleapis.com/$discovery/rest?version=v3
- Drive: googleapis.com/discovery/v1/apis/drive/v3/rest
- Docs: googleapis.com/discovery/v1/apis/docs/v1/rest
- Sheets: googleapis.com/discovery/v1/apis/sheets/v4/rest
- Slides: googleapis.com/discovery/v1/apis/slides/v1/rest

[skip-regression-check]

Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>

tools-src/google-sheets/src/lib.rs

@@ -174,7 +174,9 @@ impl exports::near::agent::tool::Guest for GoogleSheetsTool {
          (tab) management (add, delete, rename), and cell formatting (bold, colors, alignment, \
          number formats). Spreadsheet IDs are the same as Google Drive file IDs, so use the \
          google-drive tool to search for existing spreadsheets. Requires a Google OAuth token \
-         with the spreadsheets scope."
+         with the spreadsheets scope. \
+         To discover all available API operations, use http GET to fetch \
+         <https://www.googleapis.com/discovery/v1/apis/sheets/v4/rest> (public, no auth needed)."
             .to_string()
     }
 }