Fix #16397: return 503 (UNAVAILABLE) instead of 404 (UNIMPLEMENTED) w…

[LI123456mo]

Fixes part of #16397.

Problem

When pathResolver.resolve() fails to find an invoker because it was
unregistered during graceful shutdown, GrpcRequestHandlerMapping throws
a 404, which maps to gRPC UNIMPLEMENTED. Consumer-side cluster fault
tolerance treats UNIMPLEMENTED as a permanent, non-retriable error, so
it never fails over to another provider instance — turning a routine
rolling restart into a business-visible error.

Fix

When resolution fails specifically because the application is stopping
(ApplicationDeployer#isStopping()), return 503 instead of 404, which
maps to gRPC UNAVAILABLE — a retriable status consumers already know
how to fail over on.

Testing

Added GrpcRequestHandlerMappingTest#returnsServiceUnavailableWhenStoppingAndInvokerNotFound,
verifying a 503 is thrown when the app is stopping and resolution fails.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 60.87%. Comparing base (3a30432) to head (53bd596).

Files with missing lines	Patch %	Lines
...otocol/tri/h12/grpc/GrpcRequestHandlerMapping.java	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #16398      +/-   ##
============================================
- Coverage     60.87%   60.87%   -0.01%     
+ Complexity    11766    11762       -4     
============================================
  Files          1953     1953              
  Lines         89273    89278       +5     
  Branches      13473    13474       +1     
============================================
+ Hits          54346    54347       +1     
- Misses        29333    29337       +4     
  Partials       5594     5594              

Flag	Coverage Δ
integration-tests-java21	32.17% <33.33%> (+0.02%)	⬆️
integration-tests-java8	32.21% <33.33%> (+<0.01%)	⬆️
samples-tests-java21	32.19% <0.00%> (-0.01%)	⬇️
samples-tests-java8	29.84% <0.00%> (+0.04%)	⬆️
unit-tests-java11	59.14% <83.33%> (+0.02%)	⬆️
unit-tests-java17	58.60% <83.33%> (-0.04%)	⬇️
unit-tests-java21	58.63% <83.33%> (+0.02%)	⬆️
unit-tests-java25	58.56% <83.33%> (-0.01%)	⬇️
unit-tests-java8	59.16% <83.33%> (+0.02%)	⬆️

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