mgmt: Refactor to Support custom Token Provider

[kailash-b]

Changes

Refactors the ManagementClient wrapper to introduce a first-class ITokenProvider abstraction for token acquisition.

New classes:

• ITokenProvider — interface with a single GetTokenAsync(CancellationToken) method; the contract for all token providers.
• ClientCredentialsTokenProvider — implements ITokenProvider and IDisposable. Automatically acquires and caches tokens via the OAuth 2.0 client credentials grant. Tokens are cached until expiry with a 10-second leeway and refreshed lazily on the next call. Thread-safe via double-checked locking (SemaphoreSlim + Volatile/Interlocked).
• DelegateTokenProvider — implements ITokenProvider. A lightweight wrapper that invokes a user-supplied Func<CancellationToken, Task<string>> on every call. No caching. Intended for callers who manage their own token lifecycle (e.g. secret managers, external caches).

ManagementClientOptions now accepts a TokenProvider property instead of raw credentials, making the token acquisition strategy explicit and swappable.

Usage:

// Automatic token management
var client = new ManagementClient(new ManagementClientOptions
{
    Domain = "your-tenant.auth0.com",
    TokenProvider = new ClientCredentialsTokenProvider(
        "your-tenant.auth0.com", clientId, clientSecret)
});

// Custom token source
var client = new ManagementClient(new ManagementClientOptions
{
    Domain = "your-tenant.auth0.com",
    TokenProvider = new DelegateTokenProvider(ct => secretManager.GetTokenAsync(ct))
});

References

N/A

Testing

• This change adds unit test coverage

• This change adds integration test coverage

• This change has been tested on the latest version of the platform/language or why not

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 33.88%. Comparing base (c9630d0) to head (480f72a).
⚠️ Report is 2 commits behind head on v8.

Additional details and impacted files

@@           Coverage Diff           @@
##               v8     #926   +/-   ##
=======================================
  Coverage   33.87%   33.88%           
=======================================
  Files        1977     1978    +1     
  Lines       72861    72858    -3     
  Branches     4273     4271    -2     
=======================================
+ Hits        24685    24691    +6     
+ Misses      46952    46947    -5     
+ Partials     1224     1220    -4     

Flag	Coverage Δ
authIntTests	4.06% <43.18%> (+0.03%)	⬆️
mgmtIntTests	32.17% <100.00%> (+0.19%)	⬆️
unittests	0.43% <0.00%> (-0.01%)	⬇️

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