Sentinel soft-fail follow-ups: discovery failure as RedisConnectionException, late sentinel init, clear LastException

[mgravell]

Follow-ups to #3187 (AbortOnConnectFail=false for sentinel), as promised in #3271. None of this changes behaviour for abortConnect=true.

Changes

• Discovery failure surfaces as RedisConnectionException. GetSentinelMasterConnection with a config that has no endpoints of its own (the direct-API pattern: a long-lived SentinelConnect, then a per-service config carrying only ServiceName) threw AggregateException(InvalidOperationException("No nodes to consider")) from the seed connect when nothing was discovered. It now throws the discovery RedisConnectionException, as it did before Fix Sentinel connection leak and AbortOnConnectFail=false handling #3187. Measured against a live sentinel with an unknown service name.
• Late sentinel initialisation. A sentinel connection whose initial connect soft-failed never ran InitializeSentinel, because only the connect paths call it. Such a connection recovered and followed failovers, but only via the fallback polling: it never subscribed to +switch-master. Measured: subscription absent after recovery. InitializeSentinel is now idempotent and is also reached from a non-first reconfigure that detects a sentinel. The first connect still leaves it to ConnectImpl, so the happy-path ordering is unchanged.
• LastException cleared on recovery. The discovery failure recorded at connect time stayed in LastException after the connection reached its primary, and the exception factory folded it into every later "no connection available" message. Cleared in OnManagedConnectionRestored once the switch succeeds.
• Docs. Configuration.md now says what abortConnect means for sentinel connections, and that abortConnect=true is the fail-fast setting.

Tests

• New SentinelConfigTests.GetSentinelMasterConnection_NothingToSeedWith_AbortDisabled_ThrowsTheDiscoveryFailure (closed loopback port, no topology).
• SentinelTests.AbortOnConnectFailFalseRecoversOnceSentinelsAppear additionally asserts the +switch-master subscription and a null LastException after recovery; both were false before this change.
• All Sentinel* classes pass against a local primary/replica/three-sentinel topology (32 passed, 1 long-running skip). dotnet build Build.csproj -c Release /p:CI=true is clean. No public API changes.