CAMEL-23431: Migrate camel-grpc and camel-mllp tests from AvailablePortFinder to port-0 binding

[gnodet]

CAMEL-23431

Migrate camel-grpc and camel-mllp tests from AvailablePortFinder to port-0 ServerSocket binding, eliminating TOCTOU race conditions that can cause flaky BindException failures.

Changes

camel-grpc

• Add getLocalPort() to GrpcConsumer to expose the OS-assigned port after server start
• Change port validation from > 0 to >= 0 to allow port 0 (OS-assigned)
• Migrate all 14 test files to use port 0 and read back the actual port

camel-mllp

• Add getLocalPort() to MllpTcpServerConsumer to expose the OS-assigned port
• Migrate 6 test files to use port 0 and read back the actual port

Test plan

• CI passes for camel-grpc tests
• CI passes for camel-mllp tests
• No regressions in other modules

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[davsclaus]

@gnodet can you update this PR as it has merge conflicts now