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_quote_bbca.json

@@ -0,0 +1,18 @@
+[
+  {
+    "id": "bn91jc",
+    "instrumentId": "bn91jc",
+    "symbol": "BBCA",
+    "shortName": "Bank Central Asia",
+    "displayName": "Bank Central Asia Tbk",
+    "price": 9875.0,
+    "priceChange": 117.0,
+    "priceChangePercent": 1.2,
+    "pricePreviousClose": 9758.0,
+    "price52wHigh": 10250.0,
+    "price52wLow": 7800.0,
+    "accumulatedVolume": 12300000.0,
+    "averageVolume": 10000000.0,
+    "marketCap": 1215200000000000.0
+  }
+]