feature/SOF-8066 Fix: support API token auth for /users/me (list_accounts in JupyterLite)

[VsevolodX]

Problem

With ACCOUNT_ID + AUTH_TOKEN auth (JupyterLite inside the web app), client.list_accounts() raised ValueError: Access token is required to fetch user data.

Changes

• AuthContext.get_headers(): the single place that chooses the bearer header (OIDC) or the X-Account-Id/X-Auth-Token pair.
• BaseEndpoint: removed _get_bearer_headers and the header rewriting in request(). get_headers() delegates to AuthContext. Endpoint constructors are unchanged.
• APIClient._fetch_data: uses self.auth.get_headers().
• Tests: new parametrized test_auth_context.py. test_list_accounts covers both auth types.

Behavior note: an OIDC_ACCESS_TOKEN set in the environment after authenticate() is no longer picked up by _fetch_data.

Requires the server counterpart: https://github.com/mat3ra/web-app/pull/2979

Verification

40 unit tests pass, ruff clean.

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