confluent: bump twmb/avro to v1.6.0, generalize avro reference hydration

[twmb]

Summary

Three changes:

1. Bump github.com/twmb/avro to v1.6.0

v1.5.0 decodes decimal logical types to *big.Rat instead of
json.Number for *any targets, matching hamba/avro and linkedin/goavro.
It also exports RatFromBytes for CustomType callbacks and fixes
EncodeJSON json.Number coercion bugs for int/long schemas.

2. Add decimal CustomType for preserve_logical_types path

v1.5.0's *big.Rat decimal type can't pass through SetStructuredMut
(which uses json.Marshal, and *big.Rat doesn't implement
json.Marshaler). Added a decimal CustomType in
preserveLogicalTypeOpts that converts the raw []byte to
json.Number via avro.RatFromBytes at decode time.

3. Resolve avro references for arbitrary schema shapes

resolveAvroReferences previously only handled one specific schema
shape: a root that parses as a JSON array of reference names
(["Ref1", "Ref2"]), the Confluent Schema Registry convention for
"subject is a union of other subjects". Any other shape — a record
whose field type references another subject, an array whose items
reference another subject, etc. — produced the misleading error
"parsing root schema as enum".

It also had two bugs:

• Swallowed WalkReferences error. On walk failure, the function
  returned ("", nil), silently dropping the error.
• Transitive references not inlined. If the root referenced Foo
  and Foo referenced Bar, Bar remained unresolved inside the inlined Foo.

Replaced the hand-rolled hydrator with a recursive walker
hydrateAvroRefs that traverses type positions throughout the schema
tree and inlines any reference name it finds. Each named type is
inlined at most once per walk; subsequent references are left as string
name references.

Test plan

• go test ./internal/impl/confluent/... passes
• go test ./internal/impl/avro/... passes
• Existing TestAvroReferences (legacy shape) still passes
• 11 new TestHydrateAvroRefs subtests covering each supported shape

🤖 Generated with Claude Code

[claude]

Commits
LGTM

Review
Three well-scoped commits: a dependency bump (twmb/avro v1.3.4 → v1.5.0), a rewrite of resolveAvroReferences with a recursive schema walker (hydrateAvroRefs) that correctly handles record field types, array items, map values, union branches, transitive/self-referential references, and shared subgraphs, plus a decimal CustomType to maintain SetStructuredMut compatibility after the library's *big.Rat change. The old code only handled the Confluent union-of-names root shape and silently swallowed WalkReferences errors — both bugs are fixed. 11 new subtests cover each schema shape and edge case.

LGTM

[mmatczuk]

I wonder do we want to bump deps twice in one patch.

[twmb]

I wonder do we want to bump deps twice in one patch.

I can squash this if preferable! I'm trying not to release so much; migrating iceberg-go also popped up a few small edge cases I needed to improve (and I only realized across a patch and then a minor)

[claude]

Commits
LGTM

Review
Bumps twmb/avro to v1.6.0 and replaces the flat array-only reference hydrator with a recursive walker (hydrateAvroRefs) that correctly handles record field types, array items, map values, and union branches. Fixes three real bugs: swallowed WalkReferences error, refsMap population on mapSchema failure, and missing transitive reference inlining. Adds a decimal CustomType to handle the v1.5.0 breaking change (*big.Rat → json.Number). Comprehensive test coverage with 11 subtests including self-referential types, shared subgraphs, and non-substitution of non-type-position strings.

LGTM