test(agent): split server exec tests by concern

[be-student]

📌 Fixes

Fixes #23218

📝 Summary of Changes

• Split the 1,176-line server exec test file by message, WebSocket I/O, resize queue, and handler concerns.
• Keep the WebSocket fixture in a shared test helper and preserve all 37 tests.

Changes Made

• Refactored test organization without changing production behavior
• Kept imports local to each split file
• Verified the complete focused and package test suites

Checklist

• I used Codex to generate/review this code
• I have reviewed the project's contribution guidelines
• No card or demo-data behavior is changed
• Existing unit coverage is preserved
• I tested the changes locally
• All commits are signed with DCO

Screenshots or Logs

• go test ./pkg/agent/ -run 'AgentExec|AgentWS|AgentTerminal|HandleExec|ExecConstants' -count=1
• go test ./pkg/agent/ -count=1
• gofmt and git diff --check

👀 Reviewer Notes

The test-function inventory is unchanged: 37 before and after the split.

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