fix(wire): handle null/empty in Content and DisplayBlockData UnmarshalJSON

[Ricardo-M-L]

Problem

Content.UnmarshalJSON and DisplayBlockData.UnmarshalJSON switch on data[0] immediately after bytes.TrimSpace, with no handling for the JSON null literal and no guard for empty input:

func (c *Content) UnmarshalJSON(data []byte) error {
	data = bytes.TrimSpace(data)
	switch data[0] {       // panics if data is empty
	case '"':  ...
	case '[':  ...
	default:   return fmt.Errorf("invalid content type, ...")   // hit by "null"
	}
}

Two real consequences:

1. A null content field fails to parse. Content is used as a bare (non-Optional) field in several wire types (ToolResultReturnValue.Output, PromptParams.UserInput, TurnBegin.UserInput). When the peer sends null for one of them, encoding/json calls Content.UnmarshalJSON([]byte("null")), which falls into default and returns invalid content type — failing the entire message decode. By convention, a json.Unmarshaler should treat null as a no-op.
2. Empty input panics with runtime error: index out of range [0] with length 0.

Reproduced against main before the fix:

TestContentNullRepro:  unmarshal {output:null} err = invalid content type, expected one of "text" or "content_parts"
TestContentEmptyPanicRepro: panicked: runtime error: index out of range [0] with length 0

Fix

Return early on empty input or the null literal in both unmarshalers:

data = bytes.TrimSpace(data)
if len(data) == 0 || string(data) == "null" {
	return nil
}

DisplayBlockData is currently only reached through Optional[DisplayBlockData] (which already short-circuits null), but the same guard keeps it correct and consistent if it is ever used bare.

Tests

Added to wire/message_test.go:

• TestContent_UnmarshalJSON_Null — null standalone and as a struct field is a no-op.
• TestContent_UnmarshalJSON_EmptyDoesNotPanic — whitespace input no longer panics.
• TestDisplayBlockData_UnmarshalJSON_Null.

Existing behavior for strings, arrays/objects, and invalid tokens (TestContent_UnmarshalJSON_InvalidToken) is unchanged.

Verification

$ go test ./...
ok  github.com/MoonshotAI/kimi-agent-sdk/go
ok  github.com/MoonshotAI/kimi-agent-sdk/go/wire
ok  github.com/MoonshotAI/kimi-agent-sdk/go/wire/jsonrpc2
ok  github.com/MoonshotAI/kimi-agent-sdk/go/wire/transport
ok  github.com/MoonshotAI/kimi-agent-sdk/go/test/e2e
ok  github.com/MoonshotAI/kimi-agent-sdk/go/test/integration

$ go vet ./...   # clean
$ gofmt -l go/wire/message.go go/wire/message_test.go   # clean

[Ricardo-M-L]

Hi! Just a friendly ping on this PR. It's been open for a while — would appreciate a review when you get a chance. Thanks for your time!