fix(ack): synthesize union-owned discriminator on encode for codec branches

[leoafarias]

Problem

A discriminated-union codec branch whose encoder omits the union-owned discriminator could be decoded but not re-encoded — a broken round-trip:

Ack.discriminated<Map<String, Object?>>(
  discriminatorKey: 'type',
  schemas: {
    'cat': Ack.object({'name': Ack.string()}).codec<Map<String, Object?>>(
      decode: (d) => {'name': d['name']!},
      encode: (c) => {'name': c['name']},   // omits 'type' — union owns it
    ),
  },
).safeEncode({'name': 'Mittens'});   // FAILED; expected {'type':'cat','name':'Mittens'}

The effective branch injects the discriminator as a bare required literal; for a codec branch the intermediate map omits it, so ObjectSchema encode-validation fails. (This is the flutter_codec use case; the generated map-backed extension-type path is unaffected because it carries the discriminator in the map.)

Fix

Synthesize the discriminator on encode for codec branches only:

• New internal ObjectSchema.encodeOnlyDefaults map supplies a property value when the key is absent on encode.
• Set only on codec-backed branches, gated by an underCodec flag threaded through effectiveDiscriminatedBranch — flipped true only when a CodecSchema sits on the wrapper spine.
• Plain branches keep the bare required literal, so encoding a payload missing the discriminator still fails — no automatic branch selection.
• The field is excluded from ==/hashCode/toMap, so effective branches stay equal to authored twins and JSON Schema export is unchanged (discriminator stays required, no default) with zero schema-model changes.

Tests

Added: codec-omits-discriminator encode + round-trip, passthrough()+codec, multi typed-codec branch selection, nested Default/Codec gating, and a plain-branch regression lock.

Verified locally:

• dart analyze packages/ack/lib — no issues
• full ack suite — 921 passed
• ack_json_schema_builder — 49 passed
• ack_generator discriminated integration — 19 passed

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