feat(sheets): add sheets.appendRow write tool

[rohanaceres]

Summary

• Adds sheets.appendRow MCP tool to append rows to a Google Sheets spreadsheet
• Tool is gated behind the sheets write feature group (defaultEnabled: false, requires spreadsheets OAuth scope)
• Uses USER_ENTERED value input option so formulas and dates are parsed correctly

Test Plan

• Unit tests added for success case, URL-to-ID extraction, and error handling (SheetsService.test.ts)
• Run npm run test && npm run lint to verify all checks pass
• Manually verify appending a row to a spreadsheet returns correct updatedRange, updatedRows, updatedColumns, updatedCells
• Verify tool is disabled by default and only activates when sheets write feature is enabled

Issue #341

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[gemini-code-assist]

Code Review

This pull request introduces a new appendRow tool for Google Sheets, enabling the addition of multiple rows to a spreadsheet. The implementation includes the core service logic, tool registration with Zod validation, and unit tests. Review feedback suggests renaming the tool and its associated methods to appendRows to accurately reflect its multi-row capability and recommends broadening the input schema and TypeScript types to support numbers, booleans, and null values in addition to strings.