Fix Windows baseline test compatibility

[Elfsa-Miranda]

Summary

This PR fixes Windows baseline test compatibility issues without changing live trading, broker, scheduler, shell-tool, or remote API behavior.

The original Windows baseline failed in four concentrated areas:

• Loader cache tests leaked into the real Windows user cache directory because the tests patched HOME, while Windows home resolution can still use USERPROFILE.
• OAuth token cache tests asserted POSIX-style 0700 permissions, which are not directly round-trippable on Windows ACL-based filesystems.
• One parallel bench mock test depended on fork-style inheritance of monkeypatched registry state, which does not hold under Windows multiprocessing spawn.
• MCP SSE / streamable HTTP integration tests failed because loopback requests were routed through the local proxy, causing 502 Bad Gateway responses.

Changes

• Added an explicit loader cache root override for tests while preserving the production default cache path.

• Made OAuth token cache permission assertions platform-aware:

  • POSIX still checks 0700.
  • Windows checks creation, isolation, idempotence, and non-leakage without asserting POSIX mode equivalence.

• Skipped the fork-dependent parallel bench mock test on Windows only.

• Updated MCP loopback fixture setup so local 127.0.0.1 / localhost traffic bypasses environment proxies during the fixture lifecycle.

Validation

Validated on Windows PowerShell with Python 3.12.12 in an isolated virtual environment.

Targeted tests:

agent/tests/test_loader_retry_helpers.py: 20 passed
agent/tests/test_oauth_token_cache.py: 11 passed
agent/tests/test_bench_parallel.py: 7 passed, 1 skipped
MCP SSE / streamable HTTP integration: 7 passed

Full baseline:

pytest --ignore=agent/tests/e2e_backtest -q --tb=short
4701 passed, 6 skipped, 19 warnings in 173.50s

CLI check:

vibe-trading --help
exit code 0

Safety Notes

This PR does not:

• Start serve
• Start scheduler
• Connect to a real broker
• Enable live trading
• Enable VIBE_TRADING_ENABLE_SHELL_TOOLS
• Commit agent/.env
• Write real keys, tokens, or credentials into tests or logs