core: normalize service config number values

[codingkiddo]

Fixes #12815

This updates default service config validation to accept numeric values represented as Number, not only Double.

Common JSON parsers may deserialize integer-looking JSON values such as maxAttempts: 4 and backoffMultiplier: 2 as Integer, which previously caused defaultServiceConfig() to fail with IllegalArgumentException.

The values are normalized to Double when copied into the validated service config, preserving the existing internal representation expected by the service config parsing code.

Tests were updated to cover direct and nested integer numeric values.

Tested with:

./gradlew :grpc-core:test --tests io.grpc.internal.ManagedChannelImplBuilderTest \
  -PskipAndroid=true \
  -PskipCodegen=true

[linux-foundation-easycla]

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• ✅ login: codingkiddo / name: Vinod Kumar (6253f6e)