Round-tripped Firestore Timestamp fields have altered nanoseconds

### Description

**Expected Result:**
When round-tripping a `Timestamp` through Firestore, it is exactly the same before and after.

**Actual Result:**
The nanosecond field of the `Timestamp` is consistently different.

**Why Is This A Problem:**
It makes data consistency checking very difficult. It may result in spurious change notifications.

### Reproducing the issue

This simple XCTest demonstrates the problem:

```swift
struct TestStruct: Codable, Hashable {
let timestamp: Timestamp
}

func testTimestampRoundtrip() async throws {

let id = UUID().uuidString
let initial = TestStruct(timestamp: .init(date: Date.now))

try myCollection.document(id).setData(from: initial)

let roundTripped = try await myCollection.document(id).getDocument(as: TestStruct.self)

XCTAssertEqual(initial, roundTripped)
}
```

The assert will fail with an error like:

```
XCTAssertEqual failed:
("TestStruct(timestamp: )")
is not equal to
("TestStruct(timestamp: )")
```

Note that my test framework is configured to hit a local Firebase emulator instead of the real infrastructure - I don't know if that makes a difference. I also tried specifying `cache` as the source for `getDocument(as:)`, but that didn't make a difference.

This does not appear to be a problem with encoding and decoding, because this test works just fine:

```swift
func testTimestampCoding() throws {

let initial = TestStruct(timestamp: .init(date: Date.now))

let encoder = Firestore.Encoder()
let encoded = try encoder.encode(initial)

let decoder = Firestore.Decoder()
let decoded = try decoder.decode(TestStruct.self, from: encoded)

XCTAssertEqual(initial, decoded)
}
```

### Firebase SDK Version

11.7.0

### Xcode Version

16.2

### Installation Method

Swift Package Manager

### Firebase Product(s)

Firestore

### Targeted Platforms

iOS

### Relevant Log Output

```shell

```

### If using Swift Package Manager, the project's Package.resolved

Expand Package.resolved snippet

```json

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"version" : "11.2.0"
}
},
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}

```

### If using CocoaPods, the project's Podfile.lock

Expand Podfile.lock snippet

```yml

Replace this line with the contents of your Podfile.lock!

```