validate structuredQuery shape before json access in bundle decoder

[alhudz]

Firestore bundle query decoding (reached via loadBundle, where the bundle is commonly fetched from a CDN) reads parsed JSON without checking its shape, so a malformed bundle throws an uncaught nlohmann exception instead of failing the reader.

Repro: load a named query whose structuredQuery.from is a non-array, e.g. "from":"colls", or whose limit is an object with no value, e.g. "limit":{}.
Expected: the reader reports a decode failure.
Actual: DecodeCollectionSource calls get_ref<const std::vector<json>&>() with no is_array() guard and throws type_error.303; DecodeLimit calls limit_object.at("value") with no presence guard and throws out_of_range.403. Neither is caught on the decode path, so the process terminates.
Fix: guard both reads the way every sibling accessor in this file and in json_reader.cc already does, so malformed input fails the reader instead of throwing.

Added two regressions in bundle_serializer_test.cc.

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[paulb777]

/gemini review

[gemini-code-assist]

Code Review

This pull request adds validation checks to bundle_serializer.cc to ensure that the from clause is an array and that the limit object contains a valid integer value, along with corresponding unit tests. The reviewer suggested optimizing the limit object lookup by using find instead of contains and at to avoid redundant lookups.

[gemini-code-assist]

Using contains followed by multiple at calls results in redundant lookups in the JSON object. We can optimize this by using find to perform a single lookup.

Suggested change

	} else if (limit_object.is_object() && limit_object.contains("value") &&
	limit_object.at("value").is_number_integer()) {
	return limit_object.at("value").get<int32_t>();
	}
	} else if (limit_object.is_object()) {
	auto it = limit_object.find("value");
	if (it != limit_object.end() && it->is_number_integer()) {
	return it->get<int32_t>();
	}
	}