test: add Go native fuzz tests for schema generation

[mfainberg-cf]

Summary

Adds Go native fuzz testing for the schema generation pipeline. Based on the schema gen PR (#2788).

Fuzz Targets

Target	Package	What it exercises
FuzzResolveSchemaType	pkg/schema	Arbitrary TypeAnnotation trees through the recursive resolver
FuzzJSONSchema	pkg/schema	Random SchemaType trees through JSON Schema rendering
FuzzParsePredictor	pkg/schema/python	Arbitrary bytes as Python source through tree-sitter
FuzzParseTypeAnnotation	pkg/schema/python	Arbitrary return type strings in predict signatures

Running

# All targets, 30s each (default)
mise run test:fuzz

# Custom duration
FUZZTIME=5m mise run test:fuzz

# Single target
go test ./pkg/schema/ -run='^$' -fuzz=FuzzResolveSchemaType -fuzztime=60s

CI

New fuzz-go job runs on Go changes, 30s per target (~2 min total). Independent of other test jobs — only needs changes.

Approach

Byte encoder/decoder maps arbitrary fuzz corpus bytes to deterministic TypeAnnotation and SchemaType trees, giving the fuzzer structured exploration of the recursive type system rather than just random bytes.

[markphelps]

nice

[tempusfrangit]

@markphelps merged this to the bigger refactor of the schema-gen, which we still need to land. i forgot i stacked them, but it’s the right approach.

thanks for looking!