fix(sandbox): preserve zero exit code in E2B JSON stream

[ump45nose]

AgentScope-Java Version

2.0.1-SNAPSHOT

Description

Fixes #2603.

In E2bCodec.JSON mode, an explicit {"end":{"exitCode":0}} frame was lost twice because proto3 does not include default scalar values in getAllFields() and reports no scalar presence through hasField(). As a result, successful commands kept the missing-exit sentinel and failed.

This change:

• attaches the JSON end event when an integer exitCode was explicitly parsed, including zero;
• reads the proto3 default scalar from the present end event instead of checking scalar presence;
• adds a focused regression test for a zero exit code.

Related duplicate reports: #2601, #2602.

Tested with:

mvn -pl agentscope-extensions/agentscope-extensions-sandbox/agentscope-extensions-sandbox-e2b -am -Dtest=E2bEnvdProcessClientTest -Dsurefire.failIfNoSpecifiedTests=false test

Result: 8 tests passed, including jsonCodecPreservesZeroExitCode.

Checklist

Please check the following items before code is ready to be reviewed.

• Code has been formatted with mvn spotless:apply
• All targeted tests are passing
• Javadoc comments are complete and follow project conventions (no public API changes)
• Related documentation has been updated (not applicable; behavior fix with regression coverage)
• Code is ready for review

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