fix: add isAvailable check in getInvoker to prevent idle eviction race condition

[Phoebe0126]

What

Add an isAvailable() check on the fast path of DefClusterInvoker.getInvoker().

Why

When the idle scanner (scanUnusedClient) closes an RpcClient, there is a time window between client.close() and the closeFuture callback removing the invoker from invokerCache. During this window, business threads can obtain a stale invoker whose underlying transport is already shut down, resulting in Client stop errors.

How

If the cached invoker's RpcClient is no longer available (STOPPING/STOPPED state), fall through to createInvoker() which rebuilds a new RpcClient under proper synchronization.

[github-actions]

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[Phoebe0126]

I have read the CLA Document and I hereby sign the CLA

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.84151%. Comparing base (953a3c6) to head (f4b3873).

Additional details and impacted files

@@                  Coverage Diff                  @@
##                master        #140         +/-   ##
=====================================================
- Coverage     85.86041%   85.84151%   -0.01891%     
+ Complexity        4325        4321          -4     
=====================================================
  Files              436         436                 
  Lines            14371       14373          +2     
  Branches          1286        1287          +1     
=====================================================
- Hits             12339       12338          -1     
- Misses            2032        2035          +3     

Files with missing lines	Coverage Δ
...ncent/trpc/core/cluster/def/DefClusterInvoker.java	75.96154% <100.00000%> (+1.45174%)	⬆️

... and 2 files with indirect coverage changes

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