Pin InvestEngine parser failure modes — empty-on-junk + partial-match

Context: The port's graceful-failure contract was implicit in the way
each strategy returns None/[] on malformed input, but without tests it
was an accidental property that could regress silently. Codify it.

Two invariants, each backed by a fixture:

1. Junk email → empty list, never raise.
   `unparseable.eml` is a pure-marketing IE newsletter with no order
   data. All three strategies try and fail; parse_invest_engine_email
   returns []. No exception leaks.

2. Partial HTML email → intact orders only.
   `html_partial_match.eml` has two nested summary tables: one with a
   valid VUAG order, one that is missing both the ticker and "Bought N
   @ £P" rows (simulates IE dropping content mid-render). The parser
   returns just the VUAG order.

No implementation change needed — the behaviour existed as a side
effect of _try_html_summary_table returning None on missing fields.
These tests lock it down so future refactors can't quietly break it.

Test plan:
  poetry run pytest tests/providers/parsers/ -q   →  8 passed in 0.19s
  poetry run mypy broker_sync/providers/parsers/invest_engine.py tests/providers/parsers/test_invest_engine.py   →  clean
  poetry run ruff check broker_sync/providers/parsers/invest_engine.py tests/providers/parsers/test_invest_engine.py   →  All checks passed!
  poetry run yapf --diff   →  clean (no diff)

Manual verification:
- Load unparseable.eml → parse returns [].
- Load html_partial_match.eml → parse returns exactly 1 activity (VUAG).

tests/fixtures/invest_engine/unparseable.eml

@@ -0,0 +1,15 @@
+From: InvestEngine <no-reply@investengine.com>
+To: viktorbarzin@example.com
+Subject: InvestEngine newsletter
+Date: Thu, 10 Apr 2025 12:00:00 +0000
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+
+Hi Viktor,
+
+This is a newsletter, not a trade confirmation. There is no structured
+order data here — just marketing copy and a promo for a new feature we
+are rolling out. Thanks for being a customer.
+
+Cheers,
+The InvestEngine team