Clean up discriminated codec encoding

[leoafarias]

Summary

• Remove the discriminated branch encodeOnlyDefaults synthesis path from ObjectSchema.
• Make DiscriminatedObjectSchema own discriminator injection after branch encoding.
• Keep encode-time injected discriminators optional inside effective branches so codec branches can omit the discriminator while still rejecting conflicting values.
• Disable DCM rules that do not fit the current package SDK floor and schema constructor model.

Validation

• dart format --set-exit-if-changed packages/ack/lib/src/schemas/discriminated_object_schema.dart packages/ack/lib/src/schemas/object_schema.dart packages/ack/lib/src/utils/discriminated_branch_utils.dart
• dart analyze packages/ack --fatal-infos
• (cd packages/ack && dart test) - 921 tests passed

Notes

• dart run melos run analyze --no-select is blocked locally because pubspec.yaml points Melos at .fvm/flutter_sdk, which is not present in this workspace.

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