Add WithToken context override for per-request authentication

[Its-donkey]

Summary

• Adds WithToken(ctx, *Token) for per-request token overrides via context, enabling concurrent requests with different user tokens through a single client instance
• Follows the same context-override pattern as NoCacheContext
• Includes 5 new tests covering override behaviour, nil fallback, concurrency safety, and empty context
• Full documentation updates across changelog, auth reference, advanced features, quickstart, examples, FAQ, troubleshooting, and cookbook

Motivation

Endpoints like Get Channel Followers require a user token scoped to each channel (moderator:read:followers). Without per-request token overrides, users must create N separate Client instances to make concurrent requests for N different channels. WithToken solves this:

for _, ch := range channels {
    go func(ch Channel) {
        ctx := helix.WithToken(ctx, ch.UserToken)
        followers, _ := client.GetChannelFollowers(ctx, &helix.GetChannelFollowersParams{
            BroadcasterID: ch.ID,
        })
    }(channel)
}

Changes

Code:

• helix/client.go: Added WithToken, tokenFromContext, tokenContextKey. Modified doOnceWithResponse token resolution order (context > authClient > tokenProvider)
• helix/client_test.go: 5 new tests

Documentation:

• CHANGELOG.md: Added unreleased entry
• docs/auth.md: New "WithToken (Per-Request Override)" section under Token Helpers
• docs/advanced.md: New "Per-Request Token Override" section with concurrent multi-token example
• docs/quickstart.md: Added WithToken to Token Management snippet
• docs/examples/authentication.md: New "Per-Request Token Override" section with full working example
• docs/examples/batch-caching.md: New "Concurrent Requests with Different Tokens" section
• docs/faq.md: New FAQ entry "Can I use different tokens for different requests?"
• docs/troubleshooting.md: Added WithToken + Client ID mismatch pitfall note
• docs/cookbook.md: Added index entries for per-request token override

Note: PR #53 was accidentally merged into test instead of main. This PR targets main correctly and includes the same code changes plus full documentation.

[Its-donkey]

Closing - PRs should target test branch per promotion workflow.