fix(sandbox-e2b): preserve zero exit codes in envd streams

[mikemikimike]

Summary

Fixes #2793.

E2bEnvdProcessClient incorrectly relies on protobuf field presence for the proto3 scalar exit_code. That causes JSON exitCode: 0 end events to be dropped, and causes both JSON and binary streams to retain Integer.MIN_VALUE instead of the successful exit code.

This change:

• preserves an end event when JSON explicitly contains exitCode, including 0;
• reads the proto3 scalar default unconditionally from parsed end events;
• adds a regression test through the JSON stream-draining path.

Validation

• mvn -pl agentscope-extensions/agentscope-extensions-sandbox/agentscope-extensions-sandbox-e2b -am -Dspotless.check.skip=true -Dtest=E2bEnvdProcessClientTest -Dsurefire.failIfNoSpecifiedTests=false test — 8 passed
• mvn -pl agentscope-extensions/agentscope-extensions-sandbox/agentscope-extensions-sandbox-e2b spotless:check — passed
• git diff --check — passed

The Maven commands were run with JDK 21 because the repository's Spotless plugin requires Java 17+; the shell's default JAVA_HOME points to JDK 8.

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...e/extensions/sandbox/e2b/E2bEnvdProcessClient.java	33.33%	0 Missing and 2 partials ⚠️

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