length validation in KeyValuePair::encode for BytesValue

Hello folks,

while writing unit tests for the [coding](https://github.com/cloudflare/moq-rs/blob/f1b1d8bb8266ab9ff12db9d3bea3c7a962aa97b3/moq-transport/src/coding/kvp.rs#L85) module, i noticed decode validates `BytesValue` length against `u16::MAX` (Section 1.4.2), but encode doesn't. i'm not entirely sure if i'm right here (rust isnt my strongest area) but i can open a pr if you think it makes sense.

you can reproduce with this test

```rust
#[test]
fn encode_bytes_length_exceeded() {
let mut buf = BytesMut::new();

// you can just create a vector with 65535 + 1 bytes
// ignoring draft-ietf-moq-transport-15 specification:
//
// "Length: Only present when Type is odd. Specifies the length of
// the Value field in bytes. The maximum length of a value is 2^16-1
// bytes. If an endpoint receives a length larger than the maximum,
// it MUST close the session with a Protocol Violation."
let huge_bytes = vec![0u8; u16::MAX as usize + 1];
let kvp = KeyValuePair::new_bytes(1, huge_bytes);
let result = kvp.encode(&mut buf);

assert!(result.is_err(), "must rejected because bytes > 65535");
}
```

best

Start in moq-transport/src/coding/kvp.rs, focusing on KeyValuePair::encode and the existing BytesValue length check in decode. Run the supplied encode_bytes_length_exceeded reproduction with a 65536-byte value. Done means encoding an oversized BytesValue returns an error and the unit test passes.

Written by the indexing model from the issue text.