JSON conversion fails for maps with nested object values and repeated fields as first field

[tristantarrant]

Problem

ProtobufUtil.toCanonicalJSON throws IllegalStateException: Maps only have nested objects for values when a map's value is a message that contains nested structures (sub-maps, repeated fields). This was introduced in PR #411 which rewrote the JSON writer hierarchy.

Additionally, ArrayJsonWriter drops the property name when a repeated field is the first content in a non-complex object (no _type written), producing malformed JSON.

Root Cause

MapJsonWriter:

• onTag treated field number 1 as a map key even when the delegate (value message) was active
• onStartNested threw instead of forwarding to the delegate when the value message was open
• onEndNested cleared the delegate too early

ArrayJsonWriter:

• onStartNested only handled the followedByComma(lastToken()) case but not isOpen(lastToken()), so the property name was missing when a repeated field is the first content in an object

Fix

• MapJsonWriter: added valueField tracking so nested structures inside a map's message value are correctly delegated
• ArrayJsonWriter: added the isOpen(lastToken()) branch to write the property name when the repeated field is first in an object

Test plan

• testMapWithNestedObjectValues — mirrors the Keycloak authenticationSessions scenario
• testMapWithMultipleNestedObjectValues — multiple map entries with nested message values
• testRepeatedFieldFirstInObject — repeated field as first content in a non-complex nested object

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