feat: Reduce streaming DNS failures with stale-fallback DNS cache

[abelonogov-ld]

Summary

• Add CachingDns, a thread-safe OkHttp Dns wrapper that caches successful lookups (10-min TTL) and returns stale cached addresses when a fresh resolution fails — preventing UnknownHostException from killing the stream during network transitions
• Share a ConnectionPool and the DNS resolver across StreamingDataSource restarts (context switches, foreground/background toggles, network changes) via StreamingDataSourceBuilderImpl, so cached state survives data source recreation
• Explicitly set retryOnConnectionFailure(true) on the streaming OkHttpClient on all API levels

Background

We observed a high percentage of DNS failures in streaming connections. The root cause is that StreamingDataSource creates a new OkHttpClient on every start() call, and ConnectivityManager restarts the data source on every network change — exactly when DNS is most fragile. OkHttp uses Dns.SYSTEM (InetAddress.getAllByName) with no caching, and Android's system DNS cache has very short TTLs (sometimes ~2 seconds) that get cleared on network transitions.

This pattern of application-level DNS caching with stale fallback is well-established: Alibaba's HTTPDNS SDK, gRPC-Java's DnsNameResolver, and Square's own DnsOverHttps module all implement similar approaches. Google validated the pattern by adding DnsOptions.StaleDnsOptions to the Android framework in API 34.

Test plan

• Unit tests for CachingDns: fresh resolution, cache hits within TTL, TTL expiry refresh, stale fallback on failure, cold failure propagation, per-hostname isolation, expiration boundary
• Existing StreamingDataSourceTest passes (builder creates data source with new constructor args transparently)
• Verify via logs that CachingDns warns on stale fallback and that stream reconnects succeed during network changes

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Note

Medium Risk
Touches streaming network connection setup and DNS resolution behavior; incorrect caching/pooling could cause connectivity regressions or use stale IPs longer than intended.

Overview
Adds CachingDns, an OkHttp Dns wrapper that caches successful lookups with a TTL and falls back to stale cached addresses when fresh resolution fails, reducing UnknownHostException disruptions during mobile network transitions.

Updates streaming to reuse a shared DNS resolver and ConnectionPool across StreamingDataSource restarts, and wires these into the EventSource OkHttp client configuration (including enabling retryOnConnectionFailure(true)). Includes unit tests covering cache hit/expiry behavior, stale fallback, per-host caching, and eviction behavior when exceeding MAX_ENTRIES.

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