Value JSON Decoding Misclassifies String "true" and "false" as Boolean

## Location
- [pkgs/swift-google-wkt/Sources/GoogleCloudWKT/Value.swift:L91-L96](file:///usr/local/google/home/coryan/google-cloud-swift/pkgs/swift-google-wkt/Sources/GoogleCloudWKT/Value.swift#L91-L96)
- [pkgs/swift-google-wkt/Sources/GoogleCloudWKT/ProtoJSONDecoder.swift:L110-L124](file:///usr/local/google/home/coryan/google-cloud-swift/pkgs/swift-google-wkt/Sources/GoogleCloudWKT/ProtoJSONDecoder.swift#L110-L124)

## Description
When decoding a `Value` using `_ProtoJSONDecoder`, JSON strings with the literal values `"true"` or `"false"` are incorrectly decoded into `Value.bool(true)` or `Value.bool(false)` rather than `Value.string("true")` / `Value.string("false")`.

This occurs because `Value.init(from: decoder)` attempts `container.decode(Bool.self)` before `container.decode(String.self)`. In `_ProtoJSONDecoder`, `InternalSingleValueContainer.decode(Bool.self)` coerces the string values `"true"` and `"false"` to booleans to support proto relaxed numeric/boolean JSON parsing. As a result, string values in JSON dynamically typed structures lose their type fidelity.

## Steps to Reproduce / Verification
Decode a `Value` containing the JSON string `"true"` via `_ProtoJSONDecoder`:
```swift
struct Wrapped: Codable, Equatable {
let value: Value
}
let json = Data(#"{"value": "true"}"#.utf8)
let decoded = try _ProtoJSONDecoder().decode(Wrapped.self, from: json)
#expect(decoded.value == .string("true")) // Fails: received .bool(true)
```

Start with Value.swift lines 91-96 and ProtoJSONDecoder.swift lines 110-124, then run the reproduction using _ProtoJSONDecoder. Add regression coverage for JSON strings "true" and "false" and verify they decode as Value.string rather than Value.bool.

Written by the indexing model from the issue text.