feat(api): add MSN Finance as alternative data provider

Add MsnProvider implementing MarketDataProvider trait with quote,
fundamentals, and history support. Includes provider-aware config
(file/env/CLI), cache namespace isolation per provider, symbol ID
mapping via embedded TSV, OHLCV resampling, and comprehensive unit
+ integration tests with MSN fixture data.

Key changes:
- MsnProvider with quote, key-ratios, and chart endpoints
- ProviderKind enum (yahoo/msn) with config hierarchy support
- Provider-namespaced cache buckets to prevent cross-provider poisoning
- Provider-aware MockProvider loading correct fixtures per provider
- Integration tests for config round-trip and cache isolation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/fixtures/msn_chart_bbca_3mo.json

@@ -0,0 +1,22 @@
+[
+  {
+    "_p": "bn91jc",
+    "chartType": "3M",
+    "symbol": "BBCA",
+    "series": {
+      "timeStamps": [
+        "2025-01-06T00:00:00Z",
+        "2025-01-07T00:00:00Z",
+        "2025-01-08T00:00:00Z",
+        "2025-01-13T00:00:00Z",
+        "2025-01-14T00:00:00Z",
+        "2025-01-15T00:00:00Z"
+      ],
+      "prices": [9875.0, 9880.0, 9895.0, 9910.0, 9925.0, 9940.0],
+      "openPrices": [9800.0, 9850.0, 9870.0, 9890.0, 9905.0, 9920.0],
+      "pricesHigh": [9900.0, 9905.0, 9910.0, 9930.0, 9940.0, 9950.0],
+      "pricesLow": [9750.0, 9825.0, 9855.0, 9880.0, 9890.0, 9910.0],
+      "volumes": [12300000.0, 11000000.0, 11300000.0, 12500000.0, 11800000.0, 12000000.0]
+    }
+  }
+]