…py coverage + cov in CI

Closes the test-infra follow-ups deferred in JerBouma#140. All four items below
land in this single PR. Coverage 78% -> 92%, tests 32 -> 49.

## 1. `test_exchange_market_one_to_one` now uses the library

After JerBouma#140, `tests/test_equities.py` instantiates
`fd.Equities(use_local_location=True)` and the conftest regenerates
compression from CSV at import time. The library output is therefore
in sync with `database/equities.csv` for the test session, so the
consistency invariant can be expressed in the same idiom as the
surrounding tests:

```diff
-    import pandas as pd
-    from pathlib import Path
-    csv_path = Path(__file__).resolve().parents[1] / "database" / "equities.csv"
-    df = pd.read_csv(csv_path, dtype=str)
+    df = equities.select()
```

## 2. helpers.py / asset-class coverage gap (was 45% on helpers.py -> 86%)

Added small targeted tests for the previously-uncovered branches found
by `pytest --cov=financedatabase --cov-report=term-missing`. All pure
asserts (no recorder), following the `test_sec_enrichment_controller.py`
precedent.

In `tests/test_equities.py`:

| New test | Branch covered |
|---|---|
| `test_search_with_list_of_index` | `search(index=[…])` list-membership |
| `test_search_case_sensitive` | `search(case_sensitive=True)` |
| `test_search_case_sensitive_with_list` | list + case-sensitive combo (helpers.py:124-141) |
| `test_search_invalid_column_is_ignored` | `key not in data_filter.columns` warning |
| `test_to_toolkit_raises_without_financetoolkit` | `to_toolkit` ImportError path (mocked) |
| `test_to_toolkit_success_path` | `to_toolkit` success path (mocked Toolkit, helpers.py:236-265) |
| `test_init_raises_on_request_failure` | `__init__` `RequestException` path (mocked) |
| `test_module_show_options_raises_on_invalid_selection` | module-level `show_options` `ValueError` paths |
| `test_module_show_options_raises_on_request_failure` | module-level `show_options` network failure (mocked) |
| `test_module_show_options_local_location` | module-level `show_options(use_local_location=True)` (helpers.py:320, 336-341) |

In every asset-class `tests/test_<asset>.py` (cryptos, currencies,
equities, etfs, funds, indices, moneymarkets):

| New test | Branch covered |
|---|---|
| `test_select_with_invalid_value_raises` | `select(<filter>=...)` ValueError for unknown values — covers the validate-and-raise blocks unique to each asset class (e.g. `ETFs.py:83, 96, 109, 115-126`) |

The asset-class test exercises every filter column declared on that
class's `select()` signature, so adding a new filter automatically
needs a value validation when the test is updated.

## 3. Behavioural smoke assert in every asset-class `test_select`

Snapshot tests pass even when the library returns an empty DataFrame
(the snapshot, regenerated under the same broken state, would just be
empty too — and the test would pass silently). One cheap assertion per
file catches that class of regression:

```python
smoke = equities.select()
assert not smoke.empty
assert "country" in smoke.columns
```

Applied to all 7 asset-class test files.

## 4. Coverage reporting in CI

`pytest-cov` was already in `[tool.poetry.group.dev.dependencies]` but
wasn't invoked by the workflow. Wired it in:

```diff
-                    poetry run pytest tests/
+                    poetry run pytest tests/ --cov=financedatabase --cov-report=term-missing
```

Coverage now appears in the CI log; gaps become visible without local
runs. No Codecov upload — terminal report is enough for review-time
signal; uploading to a third party would add a token + service
dependency and can be a focused follow-up.

## Coverage breakdown (before -> after)

| File | Before | After |
|---|---:|---:|
| `helpers.py` | 45% | **86%** |
| `ETFs.py` | 73% | 90% |
| `Equities.py` | 88% | 94% |
| `Funds.py` | 83% | 92% |
| `Indices.py` | 89% | 93% |
| `Cryptos.py` / `Currencies.py` / `Moneymarkets.py` | 90% | 97% |
| **Total** | **78%** | **92%** |

## What this PR deliberately does NOT do (still deferred)

- `pytest-xdist` parallel execution. Tried it (`-n auto`); the
  conftest's compression-regen at import time runs once per worker,
  racing on the same files. For a ~25s suite the per-worker import
  overhead actually slows the wall time. The fix is non-trivial
  (coordinate regen across workers via a lock file) and not worth doing
  for a suite this small.
- `@pytest.mark.parametrize` / class-based grouping — would change
  pytest node IDs and break the snapshot positional naming convention.

## Test plan

- [ ] `pytest tests/` — 49 tests pass (was 32 before this PR)
- [ ] CI coverage line appears in the run log
- [ ] `black --check tests/` clean
- [ ] No snapshot files modified — only test code, config, and the
      `.github/workflows/testing.yml` step