fix: forward plain-string headers to token fetch request

[ankita10119]

Changes

What: Custom headers passed to ManagementClient (e.g. User-Agent) were not being forwarded to the internal POST /oauth/token token fetch request. They only appeared on subsequent Management API calls.

Why: TokenProvider constructed AuthenticationClient with headers: undefined, explicitly discarding all user-supplied headers. This was not intentional - the line was introduced in the initial v5 setup with no documented reason.

Fix: TokenProvider now extracts plain string headers from options.headers and forwards them to AuthenticationClient, so custom headers are consistently present on all outgoing requests including the token fetch.

Note: Only plain string header values are forwarded. Supplier functions (Fern's Record<string, Supplier<...>> type) are silently filtered out, as AuthenticationClient does not support async/dynamic header resolution.

No public API changes - this is a bug fix with no breaking changes.

References

• Internal Slack report: custom User-Agent header not sent on first request when using clientId + clientSecret with ManagementClient

Testing

• Added unit test: verifies plain string headers (e.g. User-Agent) are present on the POST /oauth/token request using nock's matchHeader
• Added unit test: verifies supplier function headers are silently filtered without errors
• All existing tests pass
• This change adds unit test coverage
• This change adds integration test coverage

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 89.73%. Comparing base (f9d6413) to head (91364d6).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1392   +/-   ##
=======================================
  Coverage   89.73%   89.73%           
=======================================
  Files         441      441           
  Lines       20799    20799           
  Branches    10146    10146           
=======================================
  Hits        18663    18663           
  Misses       2136     2136           

Flag	Coverage Δ
alltests	89.73% <100.00%> (ø)

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Files with missing lines	Coverage Δ
src/management/wrapper/token-provider.ts	100.00% <100.00%> (ø)

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