fix: default function code updates to functionRootPath

[binggg]

Summary

• make manageFunctions steer normal code updates toward functionRootPath instead of defaulting to zip/base64 mental models
• add a fast-fail message when updateFunctionCode is called without either functionRootPath or zipFile
• update demo prompts, generated tool metadata, docs, and schema assertions to reinforce the default path

Validation

• cd mcp && npm run build
• node scripts/generate-tools-json.mjs
• node scripts/generate-tools-doc.mjs
• cd mcp && ./node_modules/.bin/vitest run --config vitest.config.js ../tests/function-layer-tools.test.js

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