chore: gdb regions

[karenc-bq]

Summary

• Add support for restricting Aurora Global Database operations to a subset of accessible AWS regions (gdb_accessible_regions)
• Add configurable monitoring connection priority for the topology monitor (monitoring_connection_priority, gdb_monitoring_connection_priority)
• Wire panic-mode exit via reader-consensus / harvested connections when the writer is in an inaccessible region

Description

• GdbFailoverPlugin — failover candidates are filtered by accessible regions. In strict-writer mode, failover fails fast if the writer is in a non-accessible region.

• GdbReadWriteSplittingPlugin — writer connections to non-accessible regions are rejected. Reader candidates are filtered before applying home-region restrictions.

• GlobalAuroraTopologyMonitor — topology monitoring host workers skip non-accessible regions. The initial host's region is validated and the monitor fails fast if it is not accessible. When the writer is unreachable, the monitor exits panic mode by adopting harvested reader connections (stable-reader-topology consensus).

• AuroraInitialConnectionStrategyPlugin — host candidates are filtered by accessible regions before strategy-based selection. Filtering is delegated to the dialect via DatabaseDialect.filter_available_hosts(hosts, accessible_regions), which is a no-op for non-Global dialects and an actual region filter for GlobalAuroraMysqlDialect and GlobalAuroraPgDialect.

  Both GdbFailoverPlugin and GdbReadWriteSplittingPlugin validate at initialization that the configured home region is included in the accessible regions list when both are specified.

  New configuration parameter monitoring_connection_priority

  Comma-separated ordered list of priorities for the topology monitoring connection: strict-writer, strict-reader, writer-or-reader. Default is strict-writer. The monitor accepts any connection initially and asynchronously upgrades to a higher-priority host without blocking the monitoring loop.

  New configuration parameter gdb_monitoring_connection_priority

  GDB-specific extension supporting region-aware values: strict-writer-primary, strict-reader-primary, strict-reader-secondary, strict-writer-<region>, strict-reader-<region>, and plain <region> (any host in that region). Default is strict-writer-primary.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.