fix(chatter): let callers post real HTML via message_post(body_is_html=)

[glazperle]

Summary

Any markup passed to chatter_post arrives in the chatter as literal tags. Odoo escapes every str body handed to message_post — only a markupsafe.Markup survives, and RPC cannot carry one, so no caller of this tool can currently post a formatted message.

Odoo 17 added message_post(body_is_html=...) for exactly this case ("to be used only for RPC calls", mail/models/mail_thread.py). chatter_post now exposes a body_is_html parameter, forwards it on Odoo 17+, and omits it on 16, which stores bodies verbatim anyway.

User-facing behavior change: a new optional body_is_html parameter (default false). With it unset, behavior is byte-for-byte unchanged; the only addition is an advisory warnings entry when a body looks like markup but the flag is off.

Why opt-in rather than auto-detected

Detection cannot separate markup from prose. Both of these are valid tag syntax:

• Please forward to <a.schmidt@example.com>
• if a<b and b>c

Treating either as HTML silently deletes the bracketed text when Odoo renders the field — a lossy failure worse than the escaped-tags one it would fix. So nothing is ever converted: looks_like_html exists only to raise the warning, and its accepted false positives are pinned by a test.

body_is_html is part of the canonical payload, so an approval previewed as plain text cannot be executed as markup.

Coverage

Seven unit tests in tests/test_server.py (forwarding on 17, omission on 16, unchanged plain-text path, warning path, preview warning, token binding, gated round-trip) and a looks_like_html group in tests/test_tool_helpers.py including the deliberately accepted false positives. They use the existing _ChatterClient pattern and need no live Odoo.

Verification

• Odoo versions: 17.0 end-to-end against a live instance — the stored mail.message.body comes back byte-for-byte as sent, unescaped. 16 is covered by unit test only, since the change there is the absence of the kwarg.
• Odoo transport: XML-RPC. MCP transport: Streamable HTTP.
• Exact commands:

uv run python -m ruff check .                  # All checks passed!
uv run python -m mypy src                      # Success: no issues found in 34 source files
uv run python -m pytest                        # 925 passed, 1 deselected
uv run python -m pytest tests/test_server.py   # 219 passed

The deselected test is test_from_path_rejects_symlink_escape_within_upload_root, which needs the Windows symlink privilege and fails on this machine both before and after the change.

Checklist

• Updated documentation (README.md, docs/troubleshooting.md)
• Updated CHANGELOG.md under ## Unreleased
• No credentials or production data in the change
• Regression test that the default plain-text path gains no new kwarg and rewrites nothing

Relation to #61

No conflict expected: the tools_write.py regions are disjoint (chatter_post here, _execute_approved_write_gated there). Only tests/test_server.py and CHANGELOG.md are shared, both append-only.