feat(server): resize active RPC call capacity at runtime

[SunSi12138]

Fixes #611

Summary

• add a feature-specific UpdateCallCapacity(...) runtime API for the existing per-connection and server-wide hard RPC limits
• atomically publish both limits as one immutable admission snapshot while retaining the existing ServerCallAdmission and per-connection active-call counters
• make shrink non-preemptive and growth immediately visible to future acquisitions
• seal updates against Stop/drain under the lifecycle state gate
• keep deadline scanning valid when runtime growth exceeds the startup per-connection target
• update diagnostics/forced-stop snapshot sizing to use the runtime target without under-sizing below live usage

Validation

Added focused unit coverage for:

• per-server increase/shrink across multiple connections
• per-connection increase/shrink and connection reuse after rejection
• shrink below current usage without preemption
• invalid multi-field rollback
• repeated concurrent update/acquire/release with counters returning to zero
• drain sealing further updates

CI will provide the repository formatting, maintainability, Release build/unit suites, and deterministic allocation gate before this PR is moved out of Draft.