[fix] [sdk] variable_defs may be undefined

[othorizon]

variable_defs这个数组服务器可能没返回导致buildVariableMap 中直接 for遍历这个对象时报错。
这可能不是一个最佳性能的修复方式，但是保证了最小代码修改和尽可能好的可阅读性。

function buildVariableMap(variableDefs, variables) {
  const variableMap = {};
  const variableKeys = new Set(Object.keys(variables || {}));
  for (const def of variableDefs) {
    if (!variableKeys.has(def.key)) {
      continue;
    }
    variableMap[def.key] = {
      def,
      value: variables?.[def.key]
    };
  }
  return variableMap;
}

[CLAassistant]

All committers have signed the CLA.

[mocayo]

lgtm, thanks for your work

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@           Coverage Diff           @@
##             main      #11   +/-   ##
=======================================
  Coverage   88.60%   88.60%           
=======================================
  Files          57       57           
  Lines        3000     3000           
  Branches      632      632           
=======================================
  Hits         2658     2658           
+ Misses        342      341    -1     
- Partials        0        1    +1     

Components	Coverage Δ
@cozeloop/ai	91.53% <100.00%> (ø)
@cozeloop/langchain	82.76% <ø> (ø)

Files with missing lines	Coverage Δ
packages/cozeloop-ai/src/prompt/utils.ts	99.01% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[mocayo]

please edit your pr title to match our check, see git history for reference

[mocayo]

please edit your pr title to match our check, see git history for reference

[fix] [sdk] variable_defs may be undefined