feat: add useSocketId() hook to React, Vue and Svelte adapters

[hectorgrecco]

Summary

Adds a useSocketId() reactive hook/composable/rune to all three framework adapters, closing the gap described in #474.

The existing echo() function is already exported from all adapters and can be used directly for interceptor setup:

import { echo } from '@laravel/echo-react';

axios.interceptors.request.use((config) => {
    const socketId = echo().socketId();
    if (socketId) config.headers['X-Socket-Id'] = socketId;
    return config;
});

However, there was no reactive primitive for component usage that automatically updates when the connection reconnects and receives a new socket ID. This PR fills that gap.

New API

React

import { useSocketId } from '@laravel/echo-react';

const socketId = useSocketId(); // string | undefined, re-renders on reconnect

Vue

import { useSocketId } from '@laravel/echo-vue';

const socketId = useSocketId(); // ComputedRef<string | undefined>

Svelte

import { useSocketId } from '@laravel/echo-svelte';

const socketId = useSocketId(); // () => string | undefined (getter, Svelte 5 runes pattern)

Implementation

useSocketId() builds on the existing useConnectionStatus() in each adapter — it subscribes to onConnectionChange for free, so when the connection cycles (and a new socket ID is assigned), the value updates automatically. No new subscriptions or state management.

Test plan

• packages/react: 61 tests pass, 1 pre-existing skip
• packages/vue: 45 tests pass, 2 pre-existing skips
• packages/svelte: 32 tests pass

Closes #474