feat: support the OAuth client credentials grant

[christiansmithdeveloper]

Closes #339

Why

Zendesk is retiring API token (basic auth) authentication, and recommends the client credentials grant for server-to-server integrations where no user is present to authorise access.

The client already accepts an OAuth token (ZendeskOptions.OAuthToken → Bearer header) but has no way to obtain or renew one. That's a blocker for client credentials specifically, because:

• the grant issues no refresh token — an expired token is replaced by re-running the grant;
• access tokens expire (30 minutes by default, 48 hours max; clients created on/after 30 Apr 2026 get 30 minutes applied automatically);
• OAuthToken is assigned once at registration, so a configured token inevitably goes stale.

What

Adds first-class support for the client credentials grant:

• ClientId / ClientSecret / Scope / token-lifetime options on ZendeskOptions
• ZendeskTokenProvider — obtains tokens from POST /oauth/tokens, caches them, and renews shortly before expiry (thread-safe, single-flight)
• ZendeskOAuthDelegatingHandler — sets a current token per request on the existing zendeskApiClient named client, so a renewed token is picked up without recreating the client
• AddZendeskClientWithClientCredentials(...) registration overload

Existing basic-auth and static OAuth-token behaviour is unchanged. Version bumped 7.0.7 → 7.1.0, and the README documents the new option.

Testing

• Unit tests added for the token provider (mint, cache, renewal, single-flight concurrency, error handling, secret-not-leaked-in-errors) and the delegating handler (header set, replaced, per-request). Full suite passes (440 tests).
• Verified end-to-end against a live Zendesk instance — token minting, caching, ticket read, and ticket creation.

Note for reviewers

Zendesk's docs describe this grant as requiring grant_type and client_secret only, but the endpoint rejects the request with invalid_request: 'client_id' required. unless client_id is also sent. This is handled, and confirmed against a live instance.

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_{🤖 Implementation assisted by AI; written, reviewed, and tested by a human before submission.}