perf(mail): batch verify-on-read; test build_mail_content; addr-recall

Address PR #935 round-3 review:

- search/verification.py: rewrite _verify_mail_messages to batch by mailbox.
  get_message triggers a server-side IMAP body fetch, so per-result verify
  issued one IMAP FETCH per hit; now it calls the DB-cached list_messages once
  per mailbox (mailbox_id comes from the Qdrant payload via result.metadata)
  and intersects — O(unique mailboxes) light calls instead of O(results) IMAP.
- vector/mail_content.py: include Cc/Bcc in the indexed text so recipient
  queries match; move MAIL_SCAN_MAX_PER_MAILBOX here (shared by scanner index
  window + verifier presence window) with a note that it equals the Mail OCS
  per-request max (100), so it's a fixed constant not a config knob.
- client/mail.py: clamp list_messages limit to 1..100 at the client layer.
- tests: add test_mail_content.py (exact-layout contract for build_mail_content);
  rewrite the mail verifier tests for the batch-per-mailbox shape.

Left as-is: ValidationError isn't caught in the list-endpoint tools — consistent
with nc_notes_*/nc_deck_* and not a regression.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/unit/vector/test_mail_content.py

@@ -0,0 +1,79 @@
+"""Unit tests for the shared mail content reconstruction.
+
+``build_mail_content`` is the single source of truth for index-time and
+query-time chunk offsets; these tests pin the exact layout so a change to the
+separators or header order can't silently misalign every indexed message.
+"""
+
+import pytest
+
+from nextcloud_mcp_server.vector.mail_content import (
+    build_mail_content,
+    format_mail_addresses,
+)
+
+pytestmark = pytest.mark.unit
+
+
+def test_format_addresses_variants():
+    assert (
+        format_mail_addresses([{"label": "Alice", "email": "alice@example.com"}])
+        == "Alice <alice@example.com>"
+    )
+    # Email only, label only, label==email, and multiple joined by ", ".
+    assert format_mail_addresses([{"email": "bob@example.com"}]) == "bob@example.com"
+    assert format_mail_addresses([{"label": "Ops"}]) == "Ops"
+    assert format_mail_addresses([{"label": "x@y.z", "email": "x@y.z"}]) == "x@y.z"
+    assert format_mail_addresses(None) == ""
+    assert (
+        format_mail_addresses([{"email": "a@x.io"}, {"label": "B", "email": "b@x.io"}])
+        == "a@x.io, B <b@x.io>"
+    )
+
+
+def test_build_mail_content_plain_text_layout():
+    message = {
+        "subject": "Hello",
+        "from": [{"label": "Alice", "email": "alice@example.com"}],
+        "to": [{"email": "bob@example.com"}],
+        "hasHtmlBody": False,
+        "body": "Hi there.",
+    }
+    assert build_mail_content(message) == (
+        "Hello\nFrom: Alice <alice@example.com>\nTo: bob@example.com\n\nHi there."
+    )
+
+
+def test_build_mail_content_includes_cc_and_bcc_when_present():
+    message = {
+        "subject": "Sync",
+        "from": [{"email": "a@x.io"}],
+        "to": [{"email": "b@x.io"}],
+        "cc": [{"email": "c@x.io"}],
+        "bcc": [{"email": "d@x.io"}],
+        "hasHtmlBody": False,
+        "body": "body",
+    }
+    assert build_mail_content(message) == (
+        "Sync\nFrom: a@x.io\nTo: b@x.io\nCc: c@x.io\nBcc: d@x.io\n\nbody"
+    )
+
+
+def test_build_mail_content_converts_html_body():
+    message = {
+        "subject": "HTML",
+        "from": [{"email": "a@x.io"}],
+        "hasHtmlBody": True,
+        "body": "<p>Hello <strong>world</strong></p>",
+    }
+    result = build_mail_content(message)
+    # Header preserved; body converted to markdown (no raw tags).
+    assert result.startswith("HTML\nFrom: a@x.io\n\n")
+    assert "<p>" not in result
+    assert "world" in result
+
+
+def test_build_mail_content_tolerates_empty_fields():
+    # No subject/addresses/body (e.g. a 206 partial) -> just the blank-line +
+    # empty body, with no spurious header lines.
+    assert build_mail_content({}) == "\n\n"