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>

src/api/yahoo.rs

@@ -677,6 +677,7 @@ impl QuoteSummarySectionExt for QuoteSummarySection {
 
 #[derive(Debug, Deserialize)]
 #[serde(untagged)]
+#[allow(dead_code)]
 enum QuoteSummaryValue {
     Wrapped { raw: Option<YahooNumber> },
     Direct(YahooNumber),