server: add admission control for tenant startup wait queue

[shankeleven]

fixes #154857

When a SQL connection arrives for a default virtual cluster whose tenant server is still initializing, the server controller may wait for the tenant server to become available before accepting the connection.

Previously, an unbounded number of connections could wait concurrently, allowing a bootstrapping tenant to consume excessive resources through open TCP connections.

Add admission control to limit the number of concurrent waiters and reject excess connections immediately. Introduce metrics for wait queue observability, rate-limit timeout/rejection logs, and improve client errors to encourage retry. Also clarify documentation around DataStateReady and tenant runtime readiness.

Release note (ops change): Added cluster setting
server.controller.mux_virtual_cluster_wait.max_concurrent (default 10) to limit the number of SQL connections that may wait concurrently for a default virtual cluster to become available. Connections above the limit are rejected immediately. Added metrics under
server.controller.mux_virtual_cluster_wait.* for wait queue observability.

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