Prevent crashes when parsing malformed bundle payloads

[cherylEnkidu]

#no-changelog

Summary

This PR fixes multiple vulnerabilities in bundle deserialization where parsing malformed JSON payloads caused uncaught C++ exceptions (nlohmann::json::type_error and nlohmann::json::out_of_range) to be thrown on the Firestore worker queue, resulting in process termination.

Root Causes & Fix Details

1. Unchecked array access in DecodeCollectionSource:
   • Issue: structuredQuery.from was only checked for presence, but not verified to be an array before call to .get_ref<const std::vector<json>&>() on from_json.
   • Fix: Added a guard from_json.is_array() that fails the reader gracefully if a non-array value is supplied.
2. Missing key check in DecodeLimit:
   • Issue: Calling .at("value") on a limit object with no "value" key (e.g., "limit":{}) threw an out_of_range exception.
   • Fix: Checked limit_object.contains("value") before calling at().
3. Unchecked .contains() / .at() calls in JsonReader:
   • Issue: Downstream parsing code called .contains() and .at() on primitives when a field was expected to be an object (but supplied as a primitive), causing nlohmann to throw type errors.
   • Fix: Updated all lookup helper methods in JsonReader (Required* and Optional* methods) to first verify that the parent input value is a JSON object via .is_object().

Verification

Added regression tests to bundle_serializer_test.cc to verify that parsing failures are caught and handled gracefully (resulting in reader.status().ok() being false) rather than crashing the process:

• DecodeCollectionSourceFromNotArrayFails: Test Variant A payload (where from is a primitive string).
• DecodeLimitEmptyObjectFails: Test Variant B payload (where limit is {}).
• DecodePrimitiveAsObjectFails: Test payload where an object field (e.g. bundledQuery) is a primitive number.

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[danger-firebase-ios]

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