Default to LIFO connection reuse (server_round_robin: false) so idle reaping works under sustained load

[micahjz]

Summary

Flip the base pgcat chart to LIFO connection reuse by setting server_round_robin: false, so idle-connection reaping actually fires under sustained load.

Root cause

pgcat's code default for server_round_robin is true (src/config.rs:437, pinned by the test at src/config.rs:1866; the // False inline comment at line 324 is stale). true maps to QueueStrategy::Fifo (src/pool.rs:493).

In bb8 0.8.6, get() always pop_front() and under FIFO put() does push_back(). That makes the idle deque a true rotating queue: every checkout takes the front connection and returns it to the back. idle_start is reset only on checkin (put()), so under sustained traffic FIFO rotation touches every connection continuously and no connection ever reaches idle_timeout (30s). The reaper runs every 30s, finds nothing older than the threshold, and evicts zero. Result: connections_closed_idle_timeout = 0 while connections_created > 0, and the pool stays pinned at pool_size.

Evidence (env:prod*)

pool	txn/s	idle closes (6h)
agi_readonly	39,122	0
agi	8,134	0
finance_data	5.4	2,863

Low-traffic pools (finance_data, ~5 txn/s) reap normally because long quiet gaps let connections drift past 30s. High-traffic pools never get that gap. The running -ro pod confirms the effective value: pgcat::config: Server round robin: true.

Faithful bb8 simulation (agi_readonly-like pod)

• FIFO (current): reaped = 0, idle depth stays ~205 (matches prod)
• LIFO: reaped = 229, idle depth drains to ~12 (≈ min_idle)

Changes

1. charts/pgcat/values.yaml — add server_round_robin: false (with explanatory comment).
2. charts/pgcat/templates/secret.yaml — render the key into pgcat.toml's [general] block. The template previously omitted server_round_robin entirely, which is exactly why prod silently ran the FIFO default. Without this line the values change is inert.
3. charts/pgcat/Chart.yaml — bump 0.2.5 → 0.2.6.

Non-goals / constraints respected

• Does not touch server_lifetime (intentionally long to avoid synchronized mass expiry).
• Does not touch min_pool_size.

Rollout note

This changes the base-chart default for all deployments consuming this chart. LIFO concentrates load on a small hot working set and lets the cold tail age out — desirable here. After rollout, confirm pgcat.pgcat_databases_connections_closed_idle_timeout.count starts firing for agi/agi_readonly and sv_idle drains toward min_pool_size.