feat(email): synthetic .mbox dataset for email triage tests

[theonlychant]

Closes #848

Summary

Adds a synthetic .mbox dataset for testing the email triage agent.
The fixtures provide realistic email threads for unit and integration
testing without requiring live mailbox access.

Why GAIA needs it

The email triage agent currently has no test data to run against,
making it impossible to validate triage logic in CI.

Test plan

• tests/unit/test_agents_split.py - all tests passing

[itomek-amd]

@theonlychant — thanks for the work, and the cleanup pass that pulled connections/ back out was the right call. Requesting changes for two reasons:

1. Code doesn't parse. Two unresolved merge-conflict blocks landed in src/gaia/agents/chat/agent.py (see inline comments). python -m py_compile src/gaia/agents/chat/agent.py fails with SyntaxError: invalid decimal literal at line 1784, which is also why the Claude AI Assistant / pr-review check failed — the bot can't review a file that doesn't compile.
2. Title and scope don't match. The PR is titled feat(email): synthetic .mbox dataset for email triage tests and the body says Closes #848, but the diff bundles ~1100 lines of tool-loader work for #688/#800 and a tests/unit/test_pkce.py that imports a path (src/gaia/connections/pkce.py) the same PR deletes.

Two ways to resolve

Option A (recommended) — refocus this PR on its stated scope. Revert the tool-loader changes (src/gaia/agents/base/tool_loader.py, the tool_loader wiring in base/agent.py, the new ~260 lines in chat/agent.py, and tests/unit/test_tool_loader.py). Delete tests/unit/test_pkce.py. What's left is a clean #848 PR. Open a separate PR for the tool-loader linked to #688/#800.

Option B — keep both, but fix the merge. Resolve the conflict markers in chat/agent.py, delete tests/unit/test_pkce.py, update the title and body to declare both threads (e.g. feat: email .mbox fixtures + tool-loader for #688), and link both issues. I'd push for A — bundled PRs are harder to land and harder to revert if one half regresses.

Before re-requesting review

• python -m py_compile src/gaia/agents/chat/agent.py exits 0
• pytest tests/unit/test_synthetic_mbox.py -q is green on a clean checkout
• (If keeping any tool-loader work) pytest tests/unit/test_tool_loader.py -q is green

Happy to re-review once either option lands. The mbox fixture itself looks like a useful chunk of work.

[theonlychant]

Alright so @kovtcharov or @kovtcharov-amd and @itomek or @itomek-amd , I've addressed both issues:

Removed all tool-loader and pkce files and PR is now scoped to #848 only
Fixed the set_charset payload error in generate_mbox.py and all tests now passing

Ready for re-review when you get a chance, thanks!