Functions: callable stream fails on the server's `: ping` heartbeat

### Description

`HTTPSCallable.stream()` throws `FunctionsError` `.dataLoss` ("Unexpected format for streamed response.") as soon as the server sends an SSE comment line. `firebase-functions` (7.x) writes `: ping\n\n` whenever a streaming `onCall` has been quiet for 30 seconds (`heartbeatSeconds`, default 30), so any callable that takes longer than that without emitting a chunk fails on iOS. The Node client and the web SDK are fine with it.

The parser in `Functions.swift` only accepts lines that start with `data:`:

```swift
for try await line in stream.lines {
guard line.hasPrefix("data:") else {
continuation.finish(throwing: FunctionsError(.dataLoss, ...))
return
}
```

Comment lines (`:` prefix) are part of the SSE spec and should be skipped. Same code on `main` today.

We ran into it with a function that proxies an image generation: nothing to stream for ~50 s, then the result. Every call over 30 s failed, the ones under 30 s worked. Setting `heartbeatSeconds: null` on the function works around it.

### Reproducing the issue

1. Deploy (or run in the emulator) a streaming callable that waits before sending anything:
```ts
export const slow = onCall(async (req, res) => {
await new Promise((r) => setTimeout(r, 40_000));
res.sendChunk("hello");
return "done";
});
```
2. Call it from iOS with `Functions.functions().httpsCallable("slow").stream()`.
3. After 30 s the stream fails with `Unexpected format for streamed response.`

### Firebase SDK Version

12.17.0

### Xcode Version

27.0 (27A5252f)

### Installation Method

Swift Package Manager

### Firebase Product(s)

Functions

### Targeted Platforms

iOS

### Relevant Log Output

```
Error Domain=com.firebase.functions Code=15 "Unexpected format for streamed response."
```

Start in Functions.swift at the parser loop over stream.lines, then reproduce with the slow streaming callable described in the issue. The fix is complete when SSE comment lines beginning with ':' are ignored, data lines still stream normally, and the delayed callable no longer fails with dataLoss.

Written by the indexing model from the issue text.