Fix additionalProperties in XLR types

[justin-medeiros]

This PR fixes a an issues with additionalProperties XLR types.

AdditionalProperties merge (xlr/converters/src/ts-to-xlr.ts — handleHeritageClauses)

NavigationFlowEndState should allow any field, but in XLR additionalProperties was false because handleHeritageClauses only took additionalProperties from the extended type(s). NavigationBaseState does not allow arbitrary fields; when it was the (only) source, the merged type had additionalProperties: false even though NavigationFlowEndState should allow extra keys.

What was happening: The validator threw an error on valid navigation state JSON, e.g.:

"END_TEST": {
  "state_type": "END",
  "outcome": "TEST_OUTCOME",
  "param": "test-param"
}

Fix: We now merge the current interface’s additionalProperties with those from all extended types (already collected). When there are multiple, we combine them with an or type so the merged XLR allows any of those index signatures.

XLR before:

{
    "source": "src/index.ts",
    "name": "NavigationFlowEndState",
    "type": "object",
    "properties": {...},
    "additionalProperties": false,
   ...
}

XLR after:

{
    "source": "src/index.ts",
    "name": "NavigationFlowEndState",
    "type": "object",
    "properties": {...},
    "additionalProperties": {
        "type": "unknown"
    },
   ...
}

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