lexlint: fix panic in breakingDefs when inner schema type changes (#1348)

## Summary

- Fix panic in `breakingDefs` when comparing lexicon definitions whose
inner schema types differ (e.g. `SchemaString` vs `SchemaObject`)
- The type-change guard compared `reflect.TypeOf` on the `SchemaDef`
wrapper structs, which are always the same type, so it never triggered —
bare type assertions like `remote.Inner.(SchemaString)` then panicked
- Fix by comparing `.Inner` field types instead
- Add regression test

## Repro

Any lexicon where a field changed type between versions (e.g. a `string`
field becoming an `object`) triggers:

```
panic: interface conversion: interface {} is lexicon.SchemaObject, not lexicon.SchemaString

goroutine 1 [running]:
github.com/bluesky-social/indigo/lex/lexlint.breakingDefs(...)
	lex/lexlint/breaking.go:169
```

Author: bnewbold

lex/lexlint/breaking.go

@@ -55,13 +55,13 @@ func breakingDefs(nsid syntax.NSID, name string, local, remote lexicon.SchemaDef
 	issues := []LintIssue{}
 
 	// NOTE: in some situations this sort of change might actually be allowed?
-	if reflect.TypeOf(local) != reflect.TypeOf(remote) {
+	if reflect.TypeOf(local.Inner) != reflect.TypeOf(remote.Inner) {
 		issues = append(issues, LintIssue{
 			NSID:            nsid,
 			LintLevel:       "error",
 			LintName:        "type-change",
 			LintDescription: "schema definition type changed",
-			Message:         fmt.Sprintf("schema type changed (%s): %T != %T", name, local, remote),
+			Message:         fmt.Sprintf("schema type changed (%s): %T != %T", name, local.Inner, remote.Inner),
 		})
 		return issues
 	}

lex/lexlint/breaking_test.go

@@ -0,0 +1,24 @@
+package lexlint
+
+import (
+	"testing"
+
+	"github.com/bluesky-social/indigo/atproto/lexicon"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestBreakingDefs_InnerTypeChange(t *testing.T) {
+	assert := assert.New(t)
+
+	// When a field changes type (e.g. string -> object), breakingDefs should
+	// report a type-change error rather than panicking on a type assertion.
+	local := lexicon.SchemaDef{Inner: lexicon.SchemaString{}}
+	remote := lexicon.SchemaDef{Inner: lexicon.SchemaObject{}}
+
+	issues := breakingDefs("com.example.test", "testField", local, remote)
+
+	assert.Len(issues, 1)
+	assert.Equal("type-change", issues[0].LintName)
+	assert.Equal("error", issues[0].LintLevel)
+}