feat: support string SSE event IDs

[stareezy-1]

Summary

Fixes #1040

Problem

The SSE specification describes the event ID as a string (WHATWG: "event ID string"), but sse.Message only models it as a non-zero integer:

type Message struct {
	ID    int
	Data  any
	Retry int
}

This prevents spec-valid use cases such as UUIDs, opaque cursor tokens, composite identifiers (e.g. stream-123:456), or explicitly using 0 as an event ID — the wire output is only id: <int>, and 0 is silently omitted (if msg.ID > 0).

Fix

Add an IDString field alongside the existing ID (per the approach confirmed by the maintainers in the issue):

1. IDString, when set, is written to the id: field and takes precedence over ID.
2. ID is marked deprecated and used only when IDString is empty.
3. The generated OpenAPI id property is now oneOf: [integer, string].
4. CR/LF in IDString are stripped — they are SSE line terminators and could otherwise inject extra fields into the stream (same hardening already applied to Comment).

Tests

TestSSE/sse_string_event_ids added to sse_test.go:

Case	Verifies
IDString set	id: stream-123:456 on the wire
Both ID and IDString set	string takes precedence
Only ID set	legacy numeric ID unchanged
IDString: "0"	zero and other non-int values supported
IDString: "ab\r\ncd"	line breaks stripped, no field injection
OpenAPI document	id property is oneOf with integer and string

All existing tests continue to pass (go test -race ./...).