Improve PPCB diagnostics and failback observability

[Abhijeet Mohanty (jeet1995)]

Summary

• include per-region PPCB health state in request diagnostics and validate the data-plane diagnostics shape
• add failback backlog/recovery metrics and rate-limited recovery logging
• cache immutable PPCB diagnostic snapshots so unchanged healthy requests reuse state instead of copying the region map per request
• keep diagnostic snapshots weakly consistent while routing state remains protected by ConcurrentHashMap.compute

Validation

• PPCB manager, holder, and failback unit suites: 47 passed, 0 failed/skipped
• local Gateway V2 HTTP/2 A/B runs showed throughput within approximately 1% after snapshot caching
• JFR map-copy allocation stacks dropped to zero; sampled PPCB allocation weight dropped from 136.5 MB to 10.3 MB
• the final cache commit contains no private static state class and no partition-wide synchronized success/exception handlers

Notes

• opened as draft for review of diagnostics shape, observability, and snapshot consistency semantics
• local benchmark harness and profiling artifacts are not included in this PR

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