fix(sandbox): support nested JSON and banner prefix in AgentRun MCP response

[itxaiohanglover]

Summary

• Support nested JSON format ({"result":{"exitCode":...}}) in parseExecPayload()
• Handle markdown banner prefixes before JSON payload by locating first {
• Only treat as exec result when exitCode/exit_code is present in the resolved object

Problem

AgentRunMcpChannel.parseExecPayload() assumed flat JSON and required text to start with {. The AgentRun MCP server (protocol 2025-03-26) returns:

1. Nested format — exitCode/stdout/stderr inside a result sub-object
2. Banner prefix — markdown headers before the JSON payload

When parsing fails, the entire MCP response text (including banner) is returned as stdout, causing glob() to treat banner lines as file paths and triggering InvalidPathException on Windows.

Fixes #1908

Test Plan

• Unit test: flat JSON {"exitCode":0,"stdout":"ok","stderr":""} still parses correctly
• Unit test: nested JSON {"result":{"exitCode":1,"stdout":"","stderr":"err"}} parses correctly
• Unit test: banner + JSON ### process_exec_cmd\n...\n{"exitCode":0,...} parses correctly
• Unit test: plain text falls back to ExecResult(0, text, "")

[codecov]

Codecov Report

❌ Patch coverage is 0% with 15 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...xtensions/sandbox/agentrun/AgentRunMcpChannel.java	0.00%	15 Missing ⚠️

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[itxaiohanglover]

The codecov/patch failure is expected for fork PRs — the fork doesn't have the CODECOV_TOKEN secret configured. The actual build passes on both ubuntu and windows. This is a known issue with fork CI, not related to the code changes.