fix: refactor callback-based async flow to async/await in SAML ServiceProvider

Description:

Refactor callback-based asynchronous operations in the SAML service provider module to use modern async/await internally, without changing external behavior.

Target module:

• apps/meteor/app/meteor-accounts-saml/server/lib/ServiceProvider.ts
  Optional related utility:
• apps/meteor/app/meteor-accounts-saml/server/lib/Utils.ts

Current code uses callback-style async flows (e.g., zlib.deflateRaw and callback chaining).
Goal is to improve readability and error handling by converting internals to Promise-based patterns and async/await, while preserving public contracts and expected outputs.

Steps to reproduce:

1. Open apps/meteor/app/meteor-accounts-saml/server/lib/ServiceProvider.ts.
2. Locate callback-based async methods such as logoutResponseToUrl(...).
3. Observe callback nesting and mixed async styles.
4. Refactor internals to Promise-based flow and async/await.
5. Run related unit tests to confirm no behavior change.

Expected behavior:

• Module keeps the same external behavior and response values.
• Async logic is easier to read/maintain.
• Error paths are handled consistently.
• Existing tests continue to pass, and tests are added/updated where needed.

Actual behavior:

• Callback-based async logic is currently used in key paths, making control flow and error handling harder to follow.

Server Setup Information:

• Version of Rocket.Chat Server: N/A (development branch)
• License Type: N/A
• Number of Users: N/A
• Operating System: N/A
• Deployment Method: N/A
• Number of Running Instances: N/A
• DB Replicaset Oplog: N/A
• NodeJS Version: N/A
• MongoDB Version: N/A

Client Setup Information

• Desktop App or Browser Version: N/A
• Operating System: N/A

Additional context

This is a minor refactor task (non-architectural) intended to modernize asynchronous code style and improve maintainability.

Relevant logs:

N/A (refactor/code-quality task, not a runtime production bug)

Start in apps/meteor/app/meteor-accounts-saml/server/lib/ServiceProvider.ts, especially logoutResponseToUrl(...), and inspect the callback-based zlib.deflateRaw flow. Review apps/meteor/app/meteor-accounts-saml/server/lib/Utils.ts only if the related utility is involved, then run the related unit tests. Done means the internal flow uses async/await while external behavior, response values, error handling, and existing tests remain unchanged.